First, the bad news: 1) The one car rule is absolute, no exceptions except when a motorhome is towing a car both are allowed. 2) Your truck is out; it's too big. Nothing over 3.5 ton capacity is allowed to enter the country.
The good news: Your moped does not count as a vehicle if you are towing it.
This is copied from an official Mexican government website:
Important aspects to keep in mind for the temporary importation of a vehicle:
a) A temporary import is only possible for one vehicle at a time. [Except a motorhome towing a car or pickup can both be registered by the same person.]
b) The maximum load capacity for a temporary vehicle import is 3.5 tons, not including the weight of the vehicle.
c) You may tow with your vehicle one to three motorcycles, beach cars or dune buggies, or four-wheel motorcycles or ATVs, equivalent to the number of people traveling inside the vehicle. You must be able to provide proof of ownership for the vehicles being transported and they must be returned along with the towing or transporting vehicle.
(This post was edited by Rolly on Apr 2, 2008, 9:26 AM)