I finally got around to hooking up my 2003 LC to my travel trailer. I already had previously installed a Prodigy brake controller, after some difficulty finding the connector and cable assembly, but hadn't yet tested the whole system.
The LC and Prodigy worked well with my 16 ft 3000 lb trailer. Not so for the trailer battery hookup, at least not until I had done some rewiring.
Prior to my first trip this past weekend, I had discovered that the trailer battery would back feed through the LC trailer connection and keep some items in the LC alive with the ignition off. The odometer LCD and AC fan would continue to run and thus discharge the trailer battery while stopped for lunch en route to the campground.
Before I bought the LC last summer, I had discovered a TSB on the web that described such a problem but with the daytime running lights. Mine did not have that problem and the TSB VIN range didn't apply to mine so I thought they had fixed the problem.
I called Toyota and was told the old TSB was canceled and a new one is imminent. But I was ready to tow, so I attacked the issue with my Toyota electrical systems manual in hand. Seems that there is a relay intended to isolate the trailer battery charge line when the ignition is off. However there are other items served by the same relay, namely the AC, the rear wiper, an ECU line, and a gauge line. The battery charge line needs to be separately isolated.
So I added a relay (12v coil, 30amp contacts, Radio Shack 275-226) in series with the trailer battery charge line and fed the coil with an IGNITION ON line from junction connector J4 near the ignition switch. I suspect the upcoming Toyota TSB will do something similar.
An alternate temporary solution, if you don't need trailer battery charging while towing, is to just remove the 30A BATT CHARGE fuse located just above the left kick panel in the driver's foot well.
Dick Ballard snipped-for-privacy@att.net Beaverton, Oregon USA (near Portland)