A car that stands in a garage from October to March is, from a mouse's point of view, a warm dry nest box with its own food store and bedding. The engine bay holds heat for hours after the last run, the air filter box and the silencer are sheltered tubes, and the seat foam, carpet underfelt and hood lining are nesting material. House mice can get through a gap of 5 to 6 mm, about the diameter of a pencil, so an ordinary up-and-over door with daylight under it is no barrier at all.
The damage they do
The expensive item is the wiring loom. Rodents' front teeth grow continuously and they gnaw to wear them down, and wiring insulation, fuel hose, rubber trunking and sound-deadening felt are all chewed. A loom that has been chewed in several places is a rewire, and a chewed fuel hose on a petrol car is a fire risk. Nests in the air filter box and the exhaust tailpipe are found when the car will not start or runs badly in spring. Carpets, seats and door trims are shredded for bedding, and the smell of a mouse that has died in the heater box or under the carpet takes a long time to clear.
On an XK140 the air cleaner, the boot, the space behind the seats on the fixed-head and drophead, and the heater all shelter a nest, and the leather, the carpet underfelt and the hood lining are bedding.
Two further points worth knowing. Insurers commonly treat rodent damage as damage that happened gradually, which most motor policies exclude, and a UK Financial Ombudsman decision in 2018 did not uphold a complaint against Aviva, which had refused a claim for chewed engine wiring on exactly those grounds. And the story that modern soy-based wiring insulation attracts rodents is unproven; a US court dismissed a class action against Toyota on the point in 2018 because the owners could not show a defect, and Toyota said it knew of no scientific evidence that rodents are attracted to soy-based insulation. There is no reason to think an old loom is any safer than a new one.
What works
Keep them out of the building first. Seal gaps round pipes, cables and door frames with wire wool packed in and covered with filler or mortar, since mice will chew through foam, plastic and wood but not steel wool. Fit a brush strip or a rubber seal to the bottom of the garage door. Keep the floor clear, keep bird seed, pet food and lawn seed in metal or heavy plastic bins with lids, and do not leave bags of anything edible, including grass seed and bulbs, in the same building as the car.
Then make the car itself unwelcoming. Clean out every crumb before lay-up and leave nothing edible inside. Close the windows fully, or if you leave them cracked for ventilation, cover the gap with fine mesh. Bung the exhaust tailpipe and the air intake with a rag or a rubber bung, and hang a large label on the steering wheel saying so, because a forgotten exhaust bung is a classic spring mistake. Leave the bonnet up if the garage is secure: a cold, open engine bay is less attractive than a warm enclosed one, and you can see what is happening.
Set traps and check them. Conventional snap traps baited with peanut butter or chocolate, set against walls where mice run, are cheap and reliable, and a trap that has caught nothing in a month is useful information in itself. Check them weekly along with the car, and look for the signs: droppings the size of a grain of rice, shredded paper or felt, gnaw marks, and a musky smell. A fortnightly walk round the car with a torch, looking in the air filter box, under the scuttle, in the boot and under the seats, catches a problem while it is still one nest and not a rewire.
What is unproven
Ultrasonic repellers are widely sold and the evidence is mixed: one published single-house trial found fewer mice caught while a unit was running, but the general view of research bodies and pest control advisers is that rodents get used to the sound within days and that the devices are not a substitute for proofing and traps. Peppermint oil, dryer sheets, mothballs and cat hair are all repeated on forums; none has good evidence behind it, and naphthalene mothballs have not been legal to sell in the UK since 2008. By all means use them, but not instead of the measures above.
Poison
Rodenticide is a last resort. It is sold to the public only in small packs for use in and around buildings, it must be kept where pets and children cannot reach it, and a poisoned mouse often dies somewhere in the car, which is the smell problem again. If you have an established infestation rather than a single visitor, a professional pest controller is a better call than more bait.
Glue traps are banned for the public across Great Britain: in Wales since 17 October 2023, in England since 31 July 2024, where only licensed professional pest controllers may use them, and in Scotland since 1 July 2026, where sale and possession are banned as well. In England and Wales they are still on sale, which is not the same as being legal to use.
In spring
Before the first start, take the air filter off and look inside the box, lift the carpets and check under the scuttle and round the fuse box and loom for chewed insulation and nesting. Remove the bungs. If you find chewed wiring, trace it fully before switching on; a short to earth is cheaper to find with a meter than with a fuse.
Short version
Seal the garage with steel wool and a door strip, store nothing edible in it, bung the exhaust and intake and label the car, set snap traps and check them and the car every week or two, and treat ultrasonic devices and peppermint as extras rather than a plan.