As the title says, I picked up an 88 GTA and after rainstorms the insultation under the carpet gets wet. I am unsure where this leak could be coming from. It appears to be on both the drivers side footwell and passenger side, any advice?

Check your wiper cowl and the drains. Also look at where the toe boards section of the floor meet the fire wall. Both of these areas are common rust areas.

the ecm wiring harness that passes thru the sheet metal behind the passenger kick panel lets water in, I believe it's connect c207. That's the reason my passenger floor pan rotted out on my 86 TPI trans am. My driver side floor pan rotted out due to a missing grommet on the firewall letting water in when it rained. I would check all grommets, the cowl, t top seals, window seal, etc. It's worth it to take the seats out, roll the carpet back, and have someone run a hose over your car while you're laying in the floor pan with a flashlight looking for where the water's coming in

Specifically in the wiper cowls, check to make sure the seam sealer hasn't hardened and fallen away. Make sure the drains are clear, and see if the screens have detached from the wiper shield. If they have, I've had good luck with gorilla glue hot glue sticks reattaching them to prevent leaves and debris from falling there