radiator fan assy do not work

My 99 plymouth grand voyager fan motor died a month ago (blade could not turn--> burned wire-->overheat-->stopped until cool down than drove home). Anyway, I replaced the new radiator fan assy and also the fan relay module (mount on the firewall next to the radiator, driver side).
And the fan still not working. I hotwire the fan and it works fine. i checked the voltage before go thru the fan relay module there is 12v. but the voltage dropped when it went thru the fan relay module (it moved a tiny bit). A friend of mine (has a little car experiences) said the ground wire got shorted somewhere when the motor got burned and stopped working. And mightbe the computer module. Can someone tell me what and where is the next step i should look for and how to make the fans work.
Desperately, thanks
btw, the car runs fine. just the fans do not work. I do turn on the a/c but the a/c fan dont run at all.
T.

Your probably on the right track with the ground wire. Make sure you have a good tight, clean ground from your fan relay. If in doubt, jump the ground from the relay to a known good ground. Also are you sure it's calling for the fan to turn on when you are measuring for voltage? The fan will only run when needed. Turn your air conditioner on and see if the fan starts up. It should turn on immediately.

My fan unit has a/c and radiator fan as one unit. I did turn my a/c on (and it supposed to work) but it does not. Therefore, i tried to test and see if there is any voltage drops before and after the fan relay module because i do not know why the fans do not work at all.

the fan relay module may have 12 volts to it but it is controlled by powertrain control module?you have a dark green wire coming out of a solid state cooling fan relay if you dont have 12 volts there fan wont turn on. if you are jumping the wire for fan make sure you have it disconnected from relay or you may burn relay out. if by hotwiring the fan it works would tell me ground is ok?

The reason I mentioned jumping the ground off relay is because although the motors and relay use the same chassis ground, there is a splice connecting them together. If splice comes loose, it's possible for the motors to work when jumped but the relay will have no ground reference.

Thanks faulty585 and B33p3r for your heps. Now i turned on the a/c and tested out the voltage drops use the good ground. From my fan module relay there are 4 wires: black (i assumed it is ground), light green, dark green and a gray. The light green and a gray had 12v accept the dark green which i expected it feeds power to the fan. the output of the dark green is very small (less than a volt). Is the dark green wire supposed to have 12v? And if i do not have 12v at the dark green wire, how and where the dark green wire got feeded from? Where is the next level that i should trace to. I know very little about how the circuit works. Please hepl. Thank for all your hepls