was having problems with my Ef fairmont today so swapped the batterys over and it didnt help. Then when I put a fully charged battery into the Tj I get all lights but when you go to crank it over everything goes blank like a flat battery except the engine light stays on. From swapping the battery, could I have blown a fuse and if so what one?

turns out the cca on the battery I had in it was 420... and it was out of a fairmont! dropped the original battery in it with a cca of 520 and she is alive

I'd imagine a 420CCA fully charged battery would crank a Magna over no problems....maybe the connections were dirty or loose.

My TJ 3.5 auto cranked up just fine with a (temporary fitment only) 300 CCA battery. So connections, bad, yes?

chances are you did not screw the battery terminal on tight enough: this means your battery will be providing the right voltage (12V) but not adequate peak current (200+ amps needed to start engine).
check that its very tight and clean ;)