That's exactly how I make my LED strips on the back of SB Cruiser change brightness for braking. But it does take some quite large resistors (high watt ratings) if the lights take much current. Voltage drop across them * current thru them is the minimum watt rating size you have to use, or else burn them up. Bigger is better so they dont' get so hot in one little spot that they melt insulation/etc around them--they're going to be wasting power as heat the entire time the lighting system is turned on, except those short moments when braking.
Using two separate lights, with just a diode on the brake light version to prevent it lighting up except when brake is engaged, is more power efficient. If the second light is at least twice as bright as the taillight is, you'll get the approximate brightness difference that a car brake light gives. (total of about three times as much light during braking as just taillights). If at least one of those lights is in a slightly different place than the taillight, it's even more obvious that you're braking.
So I also have some separate strips that light up only during braking, so that the difference between taillights and braking is very clear.
(The actual taillights I'm also using have a separate wire for tail vs brake, *and* I use a second pair as just brake lights...mo' lites is betta!
)
A post about it:
https://endless-sphere.com/forums/viewtopic.php?f=2&t=67833&hilit=brake+light%2A+cruiser+resistor%2A&start=775#p1428393
It's all mounted in an ex-KVM switchbox, because I'm using the old AT-style keyboard connectors (since I already had those and a lot of cables to match form ancient computer stuff) as my trailer-lighting connection, and that also connects the rear end lighting to the system, as originally the big box on the back of the trike was intended to be removable to let me use it as a "flatbed" deck (I've just never removed it once I built it, hasn't been a need to).
The resistors I used were, I think, 50ohm 5w (maybe 10w) ceramic in metal housings, three in series IIRC, that came out of some old test equipment rackmount box (don't know what it used to be, was already just pieces when I got it).
With just the old LED strips
brake off
brake on
With the new lights too
brake off
brake on
