Changed heater core now won't start

95 jimmy change the heater core now won't start

Won't crank or won't fire? Two completely separate issues.....

I hear the fuel pump and relay in glove capartment but engine won't turn over

Leave the dome light on with doors open while changing core? Probably a dead battery....

Tried that had the battery tested and charged still the same

Basic checks next-

Is there 12v at starter solenoid purple wire when in start mode?

Try it in neutral instead of park- if automatic tranny-

Check clutch pedal switch if manual tranny.

Check battery connections at battery and ground wire to engine block.

I didn't put the ground wire back on thanks for the suggestions