Here is a simple solution:

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R1, R2, SW1, and SW2 represent the circuit in the car that you can't modify.  D1 and D2 perform a OR function so that the bottom end of R3 goes low when either or both switches are closed.  I would put D1, D2, and R3 close to where these switches are, then R4 by the PIC.

You have to assume the long line will pick up noise and transients, so R4 provides some impedance for D3 to work against to clamp the signal at the PIC to safe levels.





  [1]: https://i.sstatic.net/IKTTk.gif