I am having a problem with the interior lights in my 6 horse imperatore van.  The lights do not work however there is power to the lights.  I have tried the lights to another power source and they work.  I have 4 lights in the van and when I ground either of the middle I lose all power.  No matter what none of the lights work and while I have power and all of the grounds seems to be good.  If anyone has any help it would be appreciated.

What do you mean by grounding the "middle"?

Is the "middle" another wire?

Do you blow a fuse?

I have checked the fuses and they are all good.  I have also checked the bulbs and they are also good.  I tested the lights on another power source  and they work fine.  When I refer to the middle I am referring to the middle two lights.  For some reason when I ground the middle two lights I am losing power all together.  If I leave the middle two lights ungrounded I have power in the two lights on the end however the lights are not working.  It seems very strange to me wheich is why I am asking the questions here.  Any help or thoughts would be appreciated.  It does not make sense to me to have power and good grounds for each light and not have them turn on at all.

Are you using a meter to check for power?  If yes... You  have a poor connection in the hot lead.  And a poor ground at the "middle".  Start at the switch and work backward with a test light connected to a known good ground.  The poor connection is allowing enough voltage through to register on the meter but not enough power to light the lamps.  When you ground the "middle" you are dropping the voltage below the meter response.

My opinion of what's happening...

I am in using a voltage meter to check for power.  The strange part is the voltage when tested is the same for all four lights.  I also rewired each ground wire on the lights to ensure a good connection.  Why would the voltage be the same for all four lights and then go to 0 when the middle lights are grounded?  When I test the positive wire in the middle lights the voltage meter is the same as the two end lights.  What I cannot understand is why when I test them using the meter I have power and a good ground and then when I ground them to the light itself I lose all power.  I have also tested the live wire all the way back to the switch and the power stays constant throughout the line.  I also tested the voltage in the line for the fans which is the same voltage as the line for the lights yet the lights work when connected to the line used for the fans.  This problem has me stumped.

After a discussion with hosspuller I have discovered that dspite the fact that my meter is showing that I have voltage inside the van that there is not enough current running through the + line to power the lights.  I tested the line using a light bulb beginning at the battery then to the entrance of the live wire into the body of the van.  I have discovered that the line is good up until it enters the body and then once in the body it goes dead.  There must be a lose connection that is causing me to lose voltage inside the van.  I now have to begin searching for the connections within the body of the van or I will attempt to rewire the lights.  Either way I have a big job in front of me.  Thansk for all the help and suggestions.  It really helped me find the source of the issue.  Now I just have to find the bad wire causing the problem.

Several additional thoughts on your problem...

Some van bodies have the wall supports running vertically.  You might drop a new wire inside the wall and then, run it along the roof line till it meets an inside light wire.  Cut the old wire there, and splice to the new wire.

Alternately, attach a new wire to the end of the old wire.  Then pull the old wire out, while feeding the new wire in.