Dash lights go off at certain times on my 2001 GMC Jimmy SLT. At night when off I cannot see gauges. All other lights work. This does not happen all the time.

The problem is probably caused by the Body Control Module (BCM) malfunctioning. This module is located under the HVAC assembly (RH side under dash). If your heater core was leaking onto it, or you damaged it during your repair, that could have caused the problems.
Start by disconnecting the battery for a few minutes and then reconnecting it to see if that resets all systems. While the battery is disconnected, you can check the condition of the BCM and the three connectors (make sure they are seated properly and the contacts are not corroded). You should also scan for error codes to see if that sheds light on the problems.

check the fuse block for a blown fuse, the one that controls dash gauges should be identified in the owner's manual.

your fuel pump sending unit is bad ! in the tank !

you dont know where the dimmer is? on the left, on the headlamp switch or a small knob.

clean and lubricate the switches in the door jamb, and spray with lubricant the door latches-worst case just remove the bulbs until you can spend some time chasing it down, and just because, make certain you don't have the interior lights switched on at the dash (it happens)

While at the junk yard pick up a used insturment cluster . Cost about $20 bucks .

chevy and gmc trucks have an on going problem with this,turn the key on,and get a rubber mallot or a hammer and hit the bottom of the fuel tank a couple of times,this will unstick something in the fuel pump/sending unit.

one wire is to excite ( turn on ) the voltage regulator the other is for the gauge or warning lamp on the dash

try putting it on a battry charger over night . when boosting the newer jimmy and car you need to make sure you have a very good constion . they will full you sometime and look at your fueses insde and out ok have a good day

check all bulbs first