dicer

One wire from the battery will not make the brake lights work. And will not make the turn signals work either.
Maybe you got away with it in your state, here you would get a nice ticket.

Please explain how you can make the above work without connecting into the brake light / turn signal circuit?

Kansas

Are you addressing your comments to me? Do you understand how trailer light harnesses with built-in relays work?

When using a trailer light harness with a built-in relay, one still has to tap into the vehicle's tail, brake and turn signal signals. What these harnesses do is to power the trailer lights from the positive cable on the battery or from the engine compartment fuse box. Signals from the vehicles tail, brake and turn signal circuits act only as triggers to the trailer harness relay. This arrangement ensures that the trailer lights operate at full intensity and that bulb-out sensors do not display warnings in the vehicles instrument cluster.

I've been towing trailers for nearly 50 years and used to have a commercial license for towing so don't mess with me, LOL! Kidding, kidding, kidding.

dicer

I do understand the use of relays for that, its just that there was no mention of anything about that in previous posts, and could confuse others. Most systems I have been around there was no relays used, it was a direct connection. Not sure about now but u haul used to have special light bulbs that had wires coming out of them with connectors for quick easy connection to the tail light circuits. And yes the relay deal would be the way to go for an LS.

Kansas

Yeah, the relay box on trailer light harnesses is typically called a "converter" so I can see how people could miss that the converter box is really just a set of relays. I doubt that you can find a professional installer these days who would power trailer lights directly from the taillight wires. I asked a Uhaul installer to do that on a vehicle about 15 years ago when I was pressed for time and he told me that he was no longer allowed to do that. The biggest problem I had in the old days of powering trailer lights directly from a vehicle's ***-end was dim trailer lights - connections had to be perfect to get even barely acceptable bulb brightness. Powering trailer lights from the battery ensures maximum bulb brightness.

Of course, many SUV's including some from Toyota/Lexus come with robust circuitry designed to power trailer lights that makes it unnecessary run a separate trailer light power wire to the vehicle battery. The circuitry and associated connector(s) are sometimes standard equipment and sometimes part of a towing prep package.

Here is a link to the trailer light harnesses currently sold by etrailer for the 1990 LS400: http://www.etrailer.com/t1-1990_Lexus_LS%20400.htm The little black in the photos is the "converter" box that contains the relay function.

I can't say enough good things about etrailer - excellent service and prices. The hitch mount 4-bike rack I bought from etrailer last year for about $100 is better than some I've seen that were $300 from other companies.

lexmoose

Just ordered my draw-tite hitch and taillight converter. Later Ill have a transmission cooler installed since I plan on towing a 1800lbs tent trailer 2-3 times this summer.

Kansas

I doubt that you need a transmission cooler to tow only 1,800 pounds / 816 kg.

The official LS400 towing limits in the specs for Europe are:
Trailer with brakes: 2000 kg
Trailer w/o brakes: 750 kg