Brian,
The 2000 Marquis should be wired the same as the 2003 Patriot as very few changes were made until Monaco implemented the Roadmaster wiring. Using the 03 Patriot wiring diagrams in Coach Assist;
CB28 provides the driving light power and feeds C4-4 (sheet 33)
C-4-4 feeds C24-1, circuit 227, (sheet 19)
C24-1 feeds the Driving lights switch connector. circuit 227 (Sheet 14)
Output of the switch connector runs to the Driving lights C24-2, circuit 228. (Sheet 14)
This would seem to indicate your problem is either a broken wire or a bad connector pin connection in C4, C24 or the Driving Light switch connector. The wires have the circuit #s printed on them.
Steve