B18c gsr, just did fresh rebuild. What's my buddy do? Puts battery cables on backwards... smfh. Long story short. Lights work, fan works, I have power. When I turn the key on av and press my push button start, I hear the fuel pump humm on, and the main relay flicker on and off for a second. No clicking in the starter, took the starter over to have checked and it turns, bought a new main relay and it still does the same, bought new push button start and its circuit switch. Checked all fuses. Still good. Idk.

 

Sometimes you can have enough power to operate stuff like you said but not enough to operate the starter, have you checked battery voltage?
Have you taken a multimeter to the starter solenoid?

 

Check the battery first and make sure it is a good battery. I've never heard of what swapping battery cables does but it might not be good.

If battery is good, then the only other thing would be to check the starter cable. A wiring diagram will help

 

Didn't have the ground in right place