[f5fstop]

According to the GM wiring diagram, they power taps into Connector C5, terminal 1 at the Underhood Fuse Block. This circuit is controlled by the PRK/LP Relay and feeds through the FRT/PRK Fuse, #43.
Connector C5 is the small 8-terminal White Connector that plugs into the bottom of the connector. Probably, the wire with the marker lamp kit includes a terminal to be used.
Inserting terminal pins into these connectors can be tricky, but not as tricky as removing the dang wires.
If you look at the bottom of the connector (where the wires enter) you will see a terminal position device (piece of plastic), that looks like a comb, this slides out and the pin from the wire slides into the correct terminal hole location. Then the comb is installed. MAKE SURE TO INSERT INTO THE CORRECT CONNECTOR TERMINAL HOLE; usually it requires a special tool to remove since the connector pin will lock into place when inserted.

Therefore, if you cannot insert the wire into the connector, there may be another alternative.
I say may be, because I have not tried it, but here goes.
Connector C1, terminal 9 (Brown wire) is the power feed to the front market lamps and is also controlled by the same relay and the same fuse.
Connector C1 is the large, 47-terminal gray connector that plugs into the bottom of the fuse block.
Reason I say it may is the wire size of the brown wire should be able to handle the added current, but I cannot say this for sure.
It would be best to use the correct connector.