I had one of those on my truck before I found an original DSII harness from a 1984 truck.
If I remember correctly, that harness does not plug into the existing harness in the truck, and it uses a Chrysler-style ballast resistor block. It required trimming and splicing to make it work. It looked way too "aftermarket" for me, and I was glad to get rid of it.
Lucille: 1985 Ford F150 XLT Lariat
*Colors: Dark Canyon Red exterior, Canyon Red interior
*Engine: 5.0, CompCams 31-230-3, "Thumper" E7 heads, Edelbrock Performer intake, Autolite 4100 carburetor, DuraSpark II ignition, Thorley Tri-Y headers, Flowmaster dual exhaust, H-pipe.
*Drivetrain: AOD transmission, 3.55 gears, 2wd.