Stop in for a cup of coffee

Maybe the grounding is poor. See above TMM was asking about that.

Maybe instead of grounding the relay G terminal to the firewall, temporarily ground it to the battery neg?

Mattax said

My gut is to blame the relay, but you could test the feed through the key switch to the S2 terminal.

Click to expand...

Jumping like this would test the relay's ability - assuming terminal G has a good ground.