I wrestled with this problem on my Cutlass a year ago. I did a number of things to solve the problem. First, remove any food sources you may have stored near the car. In my case it was squirrel food. Next, I heard mothballs placed here and there deter mice. Wrap the mothballs in tissue paper to avoid damage from direct contact with the interior.

I also bought some traps. These were live traps where a trap door shuts behind them, since I didn't have the heart to kill these cute buggers. I simply released them miles away from my house in the wild where the local hawks feed.

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95 E320 Cabriolet, 169K