If I remember correctly, this is not something that can be added after market. Due to the design, the heat strip is between the blower wheel and the evaporator coil (non accessible) to the consumer.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7