Head lamps won't come on!

[Big12Tiger]

I have a 96 GMC Sierra 1500 4x4 with 170K miles. That year, the head lamps were on all the time on dim and served as the running lights. About a year ago the high beams quit working and so I could use the low beams/running lights to drive at night but I had to turn the switch to the park light setting (1st click) because if I turned the switch to the 2nd click, the running lamps went out. So now, I have nothing. No running lamps, no high beams, no low beams, just park lights, dash lights, tail, brake & turn all work.

 

Where do I start and where do I go from here?

 

Thanks in advance!