I need spark plug wiring diagram for a 1991 302 V8

" dim headlights "? None of your lights should be 'dim'. You have 4 lights.

Side Lights.
Daytime Running Lights (DRL)
Low Beam Headlights.
High Beam Headlights.

Here are the fuses, check fuse 5 and D although they covers more items than lights. Your best bet is to find a good local auto-electrician.

1994-1998 Ford Mustang Fuse Box Diagram

https://knigaproavto.ru > ... > Ford > Mustang

17 Oct 2019 - Fuse Panel Diagram Ford Mustang Mk.4 engine: 3.8L, 4.6L, 4.9L, 5.0L and 5.8L (1994, 1995, 1996, 1997, 1998)

More here

https://www.google.com/search?q=1995+ford+mustang+gt+v8+low+beam+headlight+not+working

..

head gasket is blown most likely

You would need to know if you are lacking spark or fuel when it happens. Could be a relay, the ignition module, or a list of other things.

Hi :) Here is the 2002 Ford Mustang GT wiring diagrams for sparkplug wires.
4.6 liter SOHC V-8 VIN "X"

Try registering at autozone.com and accessing their manual for your car. It might not be the greatest, but it is free.

Here ya go

1 & 5 at front of engine. Says Mustang, but should be same.

you can get it online or go too auto zone and look it up in the manual

5.0
V8 Firing Order: 1 3 7 2 6 5 4 8 Distrib. Rotation Is Counter Clock Wise
3.8
V6 Firing Order: 1 4 2 5 3 6
1 2 3
5 6 4 <<<<< Ignition Coil Pack Plug Wire Order