I am trying to help my buddy out. This is a very bizarre electrical issue.
98 Blazer
Went out at night to start the truck, it won't even click the starter, the headlights won't come on, and when you turn the headlight switch to on, the relay buzzes like hell under the hood, and when you switch it to off, the rear wiper comes on for a few seconds.
Yeah, I am not kidding.
So I checked the battery, fine
switched out the headlight relay, no change
put in a new ignition switch, no change.
I decided to call it a night and check it out the next day.
Go to start the car in the morning, starts and works just fine, headlights come on, but now, the Brake light is on for no apparent reason, and when it got the dark, the headlights didn't automatically come on.
He drives the car throughout the day and evening where it does this again in the middle of the night. It then starts fine the next day, and then failed again that evening.

So far my only theory is something with the automatic headlights. Does the ECU control this? Is it in the headlight switch? It seems like it is shorting out when the auto headlights are being told to come on. Thank you very much for any help. Ideas?

 

Take all of the battery cable connections apart and shine them up. Both positive and negative cables, both ends.

 

tried that. The car starts and runs during the day. The headlights will turn on during the day. The problem is seems to only be night related.

Does anybody have a schematic of the auto head light system?

 

On your 98, under the battery tray, is a 175 amp mega fuse. Everything except the alternator output and the starter goes through that fuse. Did you check the connections on each side of the fuse, and make sure the fuse terminals are not corroded?

A poor connection, positive or negative, can cause seemingly non-related components to operate erratically, and also cause relays to buzz Most likely a poor connection somewhere.

Might try connecting jumper cables from another vehicle. Connect positive to the 6 gauge wire in the underhood fuse panel, and connect negative to a good clean engine ground. See if it acts differently when you try to start it. If it does, you're absolutely looking for a poor connection.