The Mexican Arms Dealers are a small-time Hispanic criminal organization that onlu appear the mission Gun Runner in Grand Theft Auto: Vice City. Their leader, Pedro Garcia, is a rival gun seller to Phil Cassidy. They have no fronts or territory in Vice City but may be the remnants of the Cholos gang.
Phil Cassidy wants to eliminate his rival gun smugglers and tells Tommy Vercetti to eliminate the gang by killing Pedro Garcia and destroying the crates of arms on their Walton trucks. Once the mission is complete, guns can then be purchased at Phil Cassidy's Place in Little Haiti. After that, the Arms Dealers are presumably defeated.
An interesting note is that during the Gun Runner mission, gangs such as the Cubans, Vice City Bikers, Haitians and Patrol Invest Group will exchange fire with the M.A.D. as they pass their turf. Interestingly enough, while the Mexican Arms Dealers do not have a unique model, they do have unique dialogue.