Just bought a used Yukon with a Power Acoustik PMD-104CMX ceiling mount monitor DVD combo. When I put in a DVD it says DISC LOADING but it won't load and play the DVD.

Here, use the following link. It will take you to the troubleshooting on page 36. I hope this helps, let me know if you still have any problems.
http://www.poweracoustik.com/pa2006/manual%20and%20catalog/ceiling%20mount%20monitors/PMD-112CMX.pdf

You cant insert a dvd disc? maybe the loading mechisum is faulty. if the dvd is inserted (hear it spinning) but no picture the reader could be faulty or the easy to fix option the out wires arn't in the correct order or colour.

is it run the the head unit in the dash?

hi, you may run a disc cleaner for the optic lens might be dirty, hope this helps

Don't know if it's worth it to you, but there's a guy selling a broken unit for just shy of $70. There's no expiration date on the item, so look around before committing to buy.
Here's the link:
http://cgi.ebay.com/Power-Acoustik-PMD-100NT-POWER-but-DVD-wont-work-AS-IS_W0QQitemZ260456864971QQcmdZViewItemQQptZCar_Amplifiers?hash=item3ca4705ccb

My first question is on the main radio is there any aux, or any switch or if you plug headphones in to it do you get sound

Try the following which i managed to get from POWER ACOUSTIK technical support:

Multi-region for PTID4006T, 406T, 4005, 405, 8006T, 8004, 4003, 5000, 5800, 5300, 530.

1. Turn the unit on and insert a DVD disc. The unit will display LOADING.

2. While the DVD player is LOADING press the following keys:
SET-UP, then press 0, 0, 0, 0 key (0 four times), then ARROW DOWN key (to change the zone to region all 0), then BAND/ENTER key and SET-UP

3. To finish the procedure, push the EJECT key. The disc will eject... Wait and the DVD disc will re-load and play.

the spindal on the motor could be bad, could have bad transverse motor or even a bad laser., had to say with out seeing it do it.

I know the Power Acoustik inside and out.

Hi, I had the same problem with my PMD-100NT. I found that there was a motor inside that, I think, was moving the laser head was skipping and the head was not moving into position. I hooked a portable to the external jacks and I am just using it as a monitor. I am [email protected] if you have any questions.