here are the wiring schematics for a 68 Dart. Take a look at the diagram of the headlight switch - it has 4 or 5 different terminals on the back - each powers a different set of lights - hazards, parking, turn signals, tail light, ground, etc. The terminal that powers the front parking lights is only switched on in the "parking light" position, and turns off when the heads are switched on. The one terminal that powers the tails and marker lights is powered in both positions. If you jump the two terminals, it will now power the front parking lights in both positions. The headlights are powered by a seperate terminal, so the heads function exactly like they did before.
http://www.mymopar.com/downloads/1968/68DartA.jpg
http://www.mymopar.com/downloads/1968/68DartB.jpg