[Doddinger]

Forward the function. The variables have to be declared. Also read here:
forward OnVehicleMod(playerid, vehicleid, componentid);

https://sampforum.blast.hk/showthread.php?tid=152262

And for the other error you should give us at least the lines.

you could also try to find out how to fix that: expected token: ";", but found "]"

by yourself. That's actually a pretty little mistake and it's mostly easy to solve.