If you planning to install front LED turn signals on your 2004-2008 Ford F-150 and don't want the signal lights to flash rapidly, simply replace the stock thermal flasher unit with an LED flasher unit.
The flasher unit is located under the dash near the steering column. Put on your 4 way flashers and feel underneath the dash to help find the flasher unit. Unplug the stock unit and replace it with a TMI EL98-EP27 LED flasher unit. These cost $12.95US on eBay and work like a charm.