Might be a 68 replacement light switch, (or if you have a painless wiring harness) there are a couple of terminals that were swapped going from 67 to 68 and it causes issues like this. 67's front parking lights go off when the headlights are on and they don't have side markers, 67 parking lights were only on when the switch was in the parking light position, while the 68's did have parking lights came on in both the parking light and headlight positions. There is also a difference in the 67 and 68 ignition switches that can lead to similar issues.