The fast flashing of turn signals is caused by the flasher not having enough resistance. Need to change it out to an electronic flasher. Original flashers use amperage through a bi-metal device to open and close circuit. The electronic flashers has a circuit that causes the flasher to operate.
Verify your wiring to the new taillights. Ensure that the ground is ground, and taillights are taillights.
Bud
USAF Retired
Pace Arrow
2003 Chev Ice Road Tracker