cause of all lights going dim ?

[Hank70SS]

There are multiple ground straps on the rad core support. Headlights, voltage regulator, etc. Make sure all those are good clean solid connections. You may also want to invest in a multimeter so you can check voltage. Harbor Freight has one for 6 or 7 bucks. Not the best quality but good enough for occasional use. I would check voltage in interior of car, fuse box is good location, while idling in park then after shifting into gear. If lights dim and voltage stays pretty constant then it would be a grounding issue. If voltage drops significantly, several volts, then more likely a charging system or positive wire issue. I would want to know which one so you know where to start looking

There are 2 sides to the equation. Positive from the battery to fuse box then lights or other accessories. The other side is grounding through the chassis back to the battery. All lights and accessories are grounded to the chassis.

As far as the horn, pull the wire from the horn then take a jumper wire from horn connection and touch to battery positive post. This will at least tell you if the horn works. Have someone push the horn button and listen to the horn relay, you should be able to hear a click in the relay when the button is pushed. This is another good place to use a multimeter. Meter between wire from horn relay going to horn and a good ground. When the button is pushed you should read over 12v on the wire.

Let us know what you find out then we can give you more ideas on where to move to next. It's a process of elimination when troubleshooting electrical problems.