Flasher Help

[Farmboy444]

ok heres the scoop, we have 3 trucks at work with utility bodies on them. They all 3 came from the factory with incandescent lights, well once we get them we switch them out to LED lights for the life and the safety reason. Well they are a 2005 F-250 5.4 Gas, 2006 Chevy Express van cutaway, 6.6L Dmax, 2008 F-250 5.4 Gas and we are looking a a used 07 or 08 diesel to put a large enclosed bed on, but thats beside the matter. Each truck the flahers flash fast because the truck thinks a buld is out due to them having LED's in them, what can i do to solve this? Install some type of a resistor or a new flasher relay but i cant locate the flasher relay on the ford, dont even want to try on the chevy youd prolly have to pull the motor to get to it. Any help will be great, Thanks