Swoop is correct. Another option is to replace the factory lights with a LED version, eliminates the ground loose issue. JW Speaker offers a direct replacement

Thanks Gentleman, I will give a full report soon

kevin is the most likely scenario
i've ran into this many times on the cat's with the light bar assy's and the multifunction lights.

if you check the ground terminal in the light assy you will find it is loose. They are only riveted onto the ground plate and they do get loose over time causing the bad ground, since the ground in this circuit effects the brake, park and reverse lights it may cause erratic behaviour in any of those circuits.
You can try to remove the rivet and put a screw in it if you want but it generally is wise to just replace the light assy as the labor to repair it cost more than the light and swapping it out.
Also check the wiring in the OHG, there is a plug just inside the OHG that may be corroded. It may be located just at the base of the OHG leg where the harness enters the OHG or at the top where it comes out (all depends on the configuration and model year).

this indicates a poor ground on the area where the alarm is mounted (?on the overhead guard?) and that the current has found an easier path to ground through the alarm rather than straight through the frame. a diode and a small capacitor could act as a filter, as would fixing the ground path.

Great thanks kevin_t.
I will look at the tri-lights on the OHG

Now that I've racked my memory and thought about it a little I remember.....truck I had did have rear combination lights and one had an issue with a short in it and the brake light was dim on that side and was back feeding(shorted) and causing the BU Alarm to sound...very lightly not loud tho,like your saying yours are doing

Do the trucks that are doing this have rear combination lights(brake,Backup, turn)? I did have this problem once with one of these it was several years ago and if I remember right it was either in the wiring going up thru or by the OHG Rear pillar post or it was in the light assembly itself....if it has brake lights check to see if one is dimmer than the other....I believe it was in the light assembly and was shorted somehow to the Backup light bulb....even if it dosent have the lights in the back the wiring will be there along with the plugs and connections....look for rubbing and chafing in the harness..the back up alarm is feed by the same wire that operates the backup lights