Rock Light circuit triggered by dome lights (doors) or by dash switch.
The fuse & relay must be sized to match or exceed the lights. A common 30A Bosch/ISO relay will handle up to 6 55W bulbs using a 30A fuse. A pair of 55W bulbs only requires a 10A fuse. The 3A fuse is only necessary if the new relay is on the other side of the firewall from the dash switch.
See also:
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https://www.fleet.ford.com/truckbbas/non-html/1997/c37_39_p.pdf 
