My brake lights and turn signal lights work but tail running lights don't. I've checked and get low voltage with meter. I've bought new bulbs but they don't work. My head lights are not terribly bright, too. I tried looking for ground wire attached to frame underneath but really couldn't find anything.

Is this a ground?

Where is the ground?

 

What vehicle are we talking about??? An old crusty truck is going to have many corroded B+ and ground connections and sockets. The grounds are going to be attached to the frame or body, it just a matter of tracing them out to find the attach points which should be near the light sources.

 

Oops.....1984 f150. Wow....the ground near the light source. It seems like all wiring is in jacket or taped up. Really can't see a ground there. How could I miss that??

 

If I remember correctly........the ground will be the thin plain black wires from each lite !! (no stripes in it etc.!!)
It might run back in that group of wires....all the way thru the harness...
I'm thinking it pairs up with the other tail light harness (also the License plate wires)... & somewhere real close to where all wires they will meet into 1 harness....
The black wire (or a couple of black wires)will come out of the main harness & end up terminating to ground somewhere along that back cross member or some where pretty close to that main harness plug right @ that crossmember on the drivers side!!! !!
You can also run a individual grounds from each lite (somewhere back behind each tail light)
If memory also is right the power wire for each Tail light has a stripe in it (maybe brown +yellow stripe & green + yellow stripe the other) you can test each wire for power before running your grounds etc...........That main harness plug @ the back cross member can also be corroded as well causing bad + power!!!
Good Luck!!

 

A multimeter and a few minutes will answer a lot of questions!