I noticed my 70 challenger headlights seem dim.
After checking the alternator output, and the ground wire for the
headlight harness, I noticed there is only 8-9 volts to the lamps. Should there be 12 volts at the lights?
Any info would be appreciated

if the wiring in that year Challenger is typical for the era, you have power running from your engine compartment to your headlight switch, through your dash wiring, then to your headlights.

I don't recall the exact numbers, but think I was 2.2 volts down at the headlight sockets.

I tapped into my high and low beam power lines close to the lights, and put in relays and ran power straight from the output to them.
I have the same voltage at the headlight sockets as coming off the back of the alternator, and noticably brighter lights.

Headlight relays will make older and older style bulbs shine as bright as they can.

They also provide the power needed to get the benefits of newer style, head lamps. Many just put in new high output bulbs, but still only getting low voltage to the head lights without relays, and then wonder why they did not get good improvement.

at a fast idle I'd check the running voltage on the low and high beam wires at the bulkhead (ideally want within several tenths of what the alt output batt has) & either work toward the lights or back to the dimmer sw & even to the headlight sw for the poor connection. relays work good but w that much drop there's got to b other connections in need of attention & I'd clean em all starting w every brass connector in the bulkheads (on both sides) & NAPA has replacement brass terminals & buy a male one 725147 to clean the female ones and a small bristle brush to clean the male ones

www.danielsternlighting.com/
I did what was recommended on my '84 GMC, and with Hella headlamps, it is very bright, as in dimming a mile away. I also upgraded to 12 gauge wiring, using the stock wiring to trigger the relays. I found a fuse/relay box from some sort of front drive Charger (?), and rewired the terminals.