I'll add one more consideration. Yes, you can just move a brake controller from one vehicle to another. But one of the advantages to the built-in one is that it will be well and carefully integrated into the the increasingly complex network of computers that are to be found in modern vehicles. This has ramifications for other vehicle sub-systems. Let me give you an example from my 2012 BMW X5.
This vehicle has 27 computers scattered around. The official controller and wiring harness simply plugs into the existing harness (in the right rear fender well, NOT under the dashboard) and is highly recommended, as "things just work." The official controller is simply a Prodigy P2. If you go down to the RV store and buy a P2, then use the generic harness also available there, suddenly you find the warning system that is built into the BMW to tell you when you have a brake, turn signal, or other bulb blown goes haywire. Strange faults start bringing up spurious CEL and throwing codes. Many people have turned up on the BMW forum I frequent, having gone the cheap-at-the-RV-dealer route, now seeking help. The fix is easy: ditch the generic harness and buy the real deal, then follow the installation instructions.
BMWs may be more complex this way than Ford or Chevy or Ram trucks. But I wouldn't bet on it. What it boils down to is your choice, pick one, either simple and problem free OR cheap and troubled. Your call.
