Use "sticky" traps baited with peanut butter. If you don't want to kill the mouse, vegetable oil will free it from the sticky. I would do it sooner rather than later because he will chew electric wire insulation disabling something.
Not sure if this applies to your make / model.
I have the same problem - but only under the bonnet - of a Pug 307. Set a trap, secured with a tiewrap, and to date am catching a mouse a night (for around a fortnight). No, not the same one... Hope to run out of mice by next week.
So, traps anyway, or use a device which emits a high pitched noise to deter them. Search in all nooks and crannies for any nests,...
Chuck a cat in and leave overnight.
There is the old joke about the Landrover engineers sent to BMW to see how to improve quality. They inspected the production line, looking at all the dimensional checks and tests carried out. Once they got to the end of the line they saw an operative open the door of the BMW coming off the line and throwing a cat in.