They're not going to "void the warranty". That's not even a real thing. If your tinkering caused an issue that would normally be covered under the warranty and the dealer can show that you caused the problem they will not have to repair it under warranty.
If you add a set of lights that isn't going to affect the exhaust warranty or engine warranty. If you change the seat covers it's not going to affect the tire warranty or electrical systems warranty. (unless you destroyed the seat airbag system while doing it)
If you clip wires and the computer burns up you will have a problem (if they discover it) but the rest of the warranty still applies to the rest of the the vehicle.
Adding cruise control is pretty easy to do. Most vehicles have the connectors already in them near the throttle cable housing. Generic systems use either an abs pulse sensor or a driveshaft pickup. I used one of those for over 100k miles and never had a problem on an astro cargo van.
I understand, changing the seat covers wouldn't affect the transmission warranty, but cutting into wires which connect to the ECM might have more consequence to Ford's warranty interpretation. IMHO. ECM affects ignition advance, shift points, maybe turbo BOV, etc. See where I am heading?
I'm fairly fearless with my 99 GMC, have even hacked into the cruise control, but times have changed. Ask any NCV3 Sprinter owner if they just go cutting wires willie nillie.
But really, my question is : has anyone done it, anyone done it with something other than Rostro unit, any "stealth" alternatives that completely plug in, no cutting necessary, can be removed before a visit to the Ford Service Dept, and easily re-installed afterwards.