Changed multi function switch, bulbs, and passenger side rear light assembly (reverse, blinker, brake light). Everything works except the rear passenger side turn signal but the front passenger side is blinking fast. What is the problem?

take it to a body shop

seems you have several different problems. High beam working and no low beams=dimmer switch is bad. Reverse lamp not working=bulb socket bad,wiring broke,bad ground at back
Brake light on drivers side stuck on=brake light switch

Yes, the brake light signal on American cars is routed through the turn signal switch. A new switch should restore both your turns and your brake lights.
American cars use the same bulb filament and same wire to the rears for both brakes and turns. Press the brakes when turning and both rear brakes will light, but since one side travels through the flasher relay, that side will blink, while the other side has a steady light.

You have a broken Ground wire From Body to Frame . You Should Have a Heavy Braided Ground wire From Engine Block to Frame on Pass side, Then You will have another one that goes from firewall to engine , like at the back of the Head on passenger side to a screw to firewall.,....One of these is Broken,Loose, Corroded.

Your right rear turn signal bulb is blown.Replace the bulb and the fast blinking will stop.

The problem has to be in the multi function stalk if all of those parts were replaced correctly. This is the actual turn signal switch on the steering column.

If the front one is flashing, then it is not a multi function switch problem. We see lots of wiring issues due to water intrusion and corrosion so you need to follow the wiring to the front and check all the connectors along the way. We also see lots of TIPM problems, which is the large underhood fuse box and this is where the power for the light comes from.

check the electrical plugs under the bed,very back just behind the bumper should be a three way plug, make sure there not loose or disconnected.

Probably a broken ground wire. Check it with a test light. Make sure you have a good ground, and then stick the test light right in the socket to see if there is power. If there is power to the test light, but still none to the bulb, it's a ground wire. If there is no power, its a power wire. Just keep poking the wires, and work your way back until you find some power. Make sure you have the lights on when you're checking. Good luck.

This is a classic example of turn signal switch failure. On GM vehicles with a multi-function turn signal switch and combination stop/turn/tail lights (all rear lights are behind red colored lenses except for back-up lights) The brake light power is routed through the turn signal switch. This makes it possible for the right rear light to function as a stop light while the left rear turn signal flashes, and vice-versa. When the contacts fail to remain constant the brake lights, and turn signals can become intermittent. The proof that the brake light switch is functioning correctly is that the high center (cyclops) brake light functions properly. Wiring integrity is verified if you can prove that turn signals operate, but that can still be intermittent with this complaint, BUT if you use your hazard switch to turn on all the signals at once, you can verify wiring integrity. This is because the turn signal, and the brake light use the same circuit, and bulb filament in the rear lighting assembly. If any Hazard lights fail to function, check bulbs first, then retest. If all Hazrd light flash correctly and reliably, bulbs, sockets, and wiring have proven integrity, and the turn signal switch is the remaining component in the system which needs to be tested. Unfortunately, this switch requires disassembly of the steering column for service, testing or replacement. If you have never disassembled, and successfully reassembled a GM tilt steering column, I would reccommend professional assistance.