I recently bought a 99 LS with a lot of miles on it. Engine and tranny are in great shape and the body isn't too bad either. It had a bad battery (side post) in it so I swapped it out with one I had bought a few months ago. The one I had was top post so I had to buy the little adapters for it. The negative battery cable wasn't long enough though, so I had to get an extension piece for it. That all went easily enough. However, when I went to hook up the positive battery cable the existing end broke off. Back to the parts store I went to buy another end to crimp on to hook to my adapter. I got the little eyelet looking thing and after a battle getting the old piece off I crimped it as best I could. There are 2 cables that were crimped together on the old piece which made it nearly impossible to get into the new piece. I ended up going back to the parts store and finding a replacement end that had 2 ends on it and I wouldn't have to use the adapter on it.
Now I am having issues with my headlights not always coming on. I wouldn't think the changing of the battery would cause this, but I never had the problem before. Sometimes they come on just fine, sometimes not. I have park lights and turn signals, but no headlights. Even using the regular headlight switch to turn them on it won't work. The daytime running lights don't come on either when it has these issues. Sometimes they come on in a few seconds, sometimes a few hours, sometimes a few days.
I drive for a living, mostly at night, so this is a huge problem. I am at my wits end with this. Does anyone have any suggestions?
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Wanted to update this. I do have DRL's, but not headlights. When I turn the headlights on the DRL's go out and its only park lights.

 

I'd check the connections under the fuse panel under the hood as well as the + terminal beside it under the plastic cover. Try getting it to a "failed" headlight state & start moving wires, wiring looms etc around while watching a wall infront of the blazer to see if the headlights come on. At least if that works it gives you somewhere to start. It might even be an ignition switch issue so look up a Swartlkk post (he's the admin. here) & he has a link in his signature for the anatomy of an ignition switch.
To me it sounds like a ground/connection issue though.

 

Thanks. Working on it today, hope to get it fixed.

 

Also when you alter with main battery power + &/or - you could have a lot of issues arise... what I'm saying is make sure the connections you made to a non factory connection are as tight & strong as possible. Computers & sensors don't get along with voltage fluctuations & we have a few in our vehicles.