We insure both our vehicles (Jeep and Motorhome) through
Lewis and Lewis.
I believe that you can get a driver's policy for Mexico which provides only Third Party Liability insurance. I don't know if Lewis and Lewis offer that. Give them a phone call. You will get a real person who will have all the answers. They are a great company to work with we have been insuring through them for 12 years. Note: They close for lunch!
Also note that Third Party Liability Coverage often does NOT cover passengers in the vehicle. For example, if you are involved in an accident neither you nor your passengers will be covered for injury. Also note that a significant number of Mexican drivers/vehicles are uninsured. Your Third Party policy won't cover you here either.
Whatever policy you end up with READ IT CAREFULLY AND UNDERSTAND IT.