99 9900 odometer issue.

I replaced the speedometer in my truck because the odometer quit working. The replacement odometer resets to 0 when the key is switched off. What is causing this? Odometer works when driving but like I said before, it resets to 0 when switched off. But the trip meter resets to the mileage that was previously on it.

 

I have power on the red wire. When I turn the key on the odometer shows 549,429,then resets to zero. The trip meter shows 96,357.7.You can reset it to zero.
The odometer & trip meter both register mileage going down the road,but then both reset to the mileage I mentioned. I bought a used dash panel & swapped it out with mine. This came from a trk. with a Cat engine. Mine has a Detroit. Would this matter? Also it is from a 04-05 model where mine is a 99.
I know the odometer can be unplugged & removed,but can you buy it separately from the rest of the module? Would this fix my problem or is the memory somewhere else? Thanks.