The ECU should be under the radio. The control unit under the seat should be the automatic seat belt control unit.

more main relay
 

I found a relay (main?) next to the ecu that looks about like the one marked fuel relay under the hood. but it has 6 wire connections instead of 4.

a new wrinkle is that the code is clear when the headlights are left on but not when they're switched off.

Turning the headlights on cues the voltage regulator to put out more voltage which gives the circuit more voltage. The relay bolted to the ECU is always the main relay. Did you find the white wire with a blue tracer at that relay?

there's no blue/white wire on the relay next to the ecu
it's got black, black/pink, green, red/white, & purple. but there is a blue/white on the one under the hood. they both have a red/white, could it be that one?

I understand the key should be on but the engine off for reading the codes, so the v reg shouldn't be working (I think). I'd left the lights on after grounding the system to clear the code.

the problem with the seat belt light is fixed. the switch wasn't closing because of crud from a broken cable. it's fixed (grounded) but I had to disconnect the automatic drive motor.

Disconnect the ECU. Turn the ignition switch on and check for 12 volts at the white/blue wire at the ECU and the purge valve. The ECU grounds the solenoids to activate them so there should always be 12 volts there with the key on. If you get less than 12 volts at the ECU terminal and the purge valve terminal then check the wiring with an ohm meter. If the wiring checks out then replace the relay. The white/blue wire is the feed for the vacuum solenoids so whichever relay has the white/blue wire is the feed relay. If you get 12 volts at all locations then the ECU is the culprit.