After noodling with two SPDT relays, which does, I'll be changing to a DPDT to better control the swap between Door Assist and regular operation. Here is the circuit I'll be using:
One bummer about using this is that the BCM output for interior lights does not provide a signal when the dome lights are manually turned on, only with the door. Not a big deal.
Basically, the Electrolytic capacitor is there to keep the relay coil happy. You'll get a lot of chattering/buzzing from the PWM signal otherwise, which is just premature wear (and also a bit terrifying). You can probably use a bigger cap, but the 220uF is working fine with the SPDT I have in there now (except for door signal). There's a diode in there to protect the BCM from the capacitor discharge.
This is the circuit I currently am using, but it's a little overcomplicated when the DPDT package should function better with the isolated Door Assist output. As I mentioned, this bottom one still has issues with the Door Assist signal. A larger cap (1000uF) may work, but I opted to isolate the signal instead using the DPDT relay which is on it's way. An RC circuit may be necessary to smooth out the PWM signal with it, rather than just a bigger capacitor, but the thing was starting to become a pain to manage with tall the jumpers and connectors.
if you do try this, make sure you do not hook up the capacitors backwards as they will blow out the top if you short them out.