I am going to try to enter Mexico(I am from USA) in March after not being there for 3-4 years. Last time I tried to enter, they showed that I have a bike in the country(ok...I didn't check out properly once...). Since I was riding a different bike, I could not prove that I did not leave the past bike in Mexico. They would not let me back in. I've tried to find the bike that I had in Mexico(I sold it a few years ago) but the local DOT can't find it anywhere so I can't prove that its in US. Any suggestions on how to get this resolved? Thanks in advance....

To fix your problem legally, I'd jump back into the DMV and try to have it located. Then get a letter stating that Joe Blow in MN owns it.

I screwed up many years ago by not retiring the permit for a truck. Ignorance was my excuse then. Several years later I was refused another permit because of this when trying to enter on a moto. I'd sold that truck to my son. This happened at either the Del Rio or Eagle Pass border, I forget....
Anyway, I finally spoke to 'some guy' in the customs office (not the counter service) and explained what had happened, even offering to call my son there, in his office, on my cell phone. We talked a little bit, I was very nice, apologetic, gracious, worried, etc. He typed a letter stating that the vehicle was in the US and not left in Mexico and told me to give it to the 'counter guy' (for lack of the correct title). As he was finishing his typing, and looking at the typewriter keys, he slowly said..."this is a little white lie". I was very lucky and my delay was maybe 1 hour, not the possible several days because of needing official DMV papers.

You could try to fake some paperwork...
Bill of Sale (looks old)
DMV letter, current owner registration, etc....
Official accident report, salvage from insurance, etc

My first inclination is to suggest South Texas borders because there are so many, even in individual towns (Monty Hall: Bridge 1, 2 or 3), where you can jump to another if you get nixed at one.

I'll also mention there are lawyers in the US border towns that furnish paperwork accepted by the Mex side regarding citizenship and other things. Large border towns, such as Laredo, will have dozens listed in the Yellow Pages, and also listed online. I've never tried to use them for this purpose, vehicle permit problems, but it might be worth a try. Showing up in person might make them more comrfortable than fielding a phone call. A stamp/seal/legal name, etc can go a long way.

Just a few ideas...