S4 Turn Signals in an S5
 

I have a 1990 FC with a bad driver's side front turn signal assembly.

Saw a good deal on S4 front turn signal assemblies and was wondering if anyone has done this swap and know if they will be compatible with S5 wiring and the bumper.

wiring yes (100% verified), bumper I don't think so

the s5s are slimmer then s4s, wiring identical
so in your case a s4 will not physically fit into the opening

I accidentally bought an s4 turn signal for the passenger front. The s4 wires up but it does not actually fit into the mounting. It's significantly smaller.

s5 are slimmer and longer s4 are shorter and wider

They Do not Fit.
and if you have an aftermarket Fiberglass bumper,most if them require an s5 turn signal assembly.

Picked up a junk signal for an S4 for very cheap. I am going to run through inspection with the S4 housing then try to transfer the wiring into my S5 assembly i'll post if it works or not.

what did you need just the bulb assembly or the whole housing?

I needed the whole housing but I am going to see if I can just transfer the electrical parts to the S5 housing.