89' jeep overheating

Sounds like a similar set up as my 88 Jetta had, the "overflow" bottle was/is part of the active cooling system, a certain amount of fluid is diverted and sent back to the bottle, likewise there is also a continuous flow of fluid from the bottle into the system. I believe this is what they call a closed system (?). try looking here for some ideas.

what kind of Jeep?

The proper term for this cooling system is a 'closed system', better known a a PITA! What you describe sounds like a Cherokee (XJ) or Commanche (MJ) with the 4.0L engine.

Two issues I've had that both led to overheating: The cap on the bottle is the pressure cap for the system and it's a royal pain to get the air out of the system when you refill it. About the only answers to the first issue is to either buy a new resevoir to get a new cap or replace the radiator and convert the cooling system to a sensible open system. I've never found the cap separately but don't dispair - you'll need the new bottle soon enough anyway.

As for the fill, you can usually get the air out by filling as far as it will go, starting the engine and run it until you see the water rise in the tank then shut it down and let it cool. Repeat until it stabilizes and no longer spits bubbles out of the top tube into the tank. Another way is to pull the temp sensor at the rear of the head and fill to overflow. I usually do best by parking on a good slope, nose up, and filling it there so that the air rises to the front of the engine. As long as it's spitting bubbles into the top tube, you have air in the system and it's going to run hot.

You can find the drain for the radiator (assuming I'm right about the XJ/MJ) on the lower passenger side corner of the radiator. You have to pull the grill insert to get at it (8 screws) and it's going to make a mess. I replaced the plug with a draincock that had enough of an extension that I could get a short piece of hose on so that I could direct the flow into a bucket.