My faithful 93 Grand Marquis has developed an odd problem When the headlights are on the wipers won’t work and vice versa.
This certainly is odd. The only place where these circuits come close to each other is in the Multifunction switch (on the turn signal stalk). I can’t for the life of me figure out how this could happen, though. Post it on www.crownvic.net and see if anyone else has seen this issue.
The multi-switch seems a likely suspect as beadsandbeads said. You might have some weird grounding problem or possible a blown fuse with the wipers somehow finding ground back through the headlights or something like that. Check the fuses carefully. They can look good and still be bad. It is best to test them with a meter.
If all the fuses are good, check for loose ground wires/straps.
I think the problem may be due to a bad ground also. Check the bond to the chassis near the battery and in the dash area.