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Just picked up a 76 530i and the low beams do not work. I replaced the headlight switch with a known good one from my e21 and replaced the headlights to no avail. They will come on with the high beams just not by themselves. Any suggestions on where to start?

Check / clean fuses and fuse contacts. Check relay in fusebox, eg. by replacing low beam relay with high beam relay.

I should have stated in my original post that I have changed the fuses and swapped the relays around. While digging under the hood last night I noticed that one of the high beams only has two prongs and not three. I will swap in a three prong high beam and see if that does the trick

In that case, can you hear the relay click when you switch on the low beams?
If so, check if you have 12V supply to the relay permanent 12V contact feeding the lights and if you have some resistive load (=bulbs) on the wire towards the lights.

Three prong lights are high / low beams, outer position.
Two prong lights are High beams only, inboard of the 'lows.'

You may have bad bulbs, burned out low filament on both sides ... hey, it could happen.

Run wires from the battery (or a charger) to the bulbs, you will know if the filaments work. Use the connector for the outer lights to tell which terminal is for ground, low and high.

Ground prong is a brown wire.
Low beam is Yellow/blue (or Yellow/white) and high is White/blue (or white/red).

Marker stripes vary in color left and right from model to model, but the main color is ALWAYS White for high, Yellow for low.

Hope this helps.