The power drop harness that you bought will plug right into the power drop in your car. In the pictures posted by Steve you will see the black plug that it goes into.
The harness makes things a little easier with gauges but honestly you're going to be splicing a lot of wires for gauges so the extra time for the 3 wires going to the power drop isn't worth the money IMO.
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1998 Pontiac Grand Prix GT
WizAired CAI, Impala 12" Brake Upgrade, ZZP Alternator Rewire and Voltage Booster, Rotated Dogbone Mounts, Kicker KS690 6x9s, JBL GTO427 4s, JBL P652S 6.5s
2001 Ford F150 XLT SuperCab
Stock, LineX, 212K