Tail light complete replacement-99'GCL

I had to buy a new driver's side tail light whole unit, got it on Ebay for $60, delivered. I was able to get the electrical connections out, but it's a real bear to put back in. I'm afraid of breaking either the connector or new light unit.

There's a red tab that I got out and seems to hold the connector to the unit AFTER it's installed. It looks like an easy insert, but won't go past the holding posts, is there a secret?

Sorry, I don't have a service manual, but I know what to ask for Xmas now.

EDIT: It seems the prongs on the new unit looks to be a bit thicker. I was able to slide the connector to the old unit real easy. The seller has sold thousands of items with a current 99.8% great rating. I guess I'll have to call them in the morning.