Wiring Automatic Headlights for Wipers with the DEI 545t

I recently bought an '07 Prius Base--the Base wasn't what I would have preferred, but the price was right, and I added $500 to the auto loan to upgrade the stereo and a few other things. One of the additions was the Directed Electronics 545t Nite-Lite automatic headlights control.

The installation for the most part went fine. I wired the power wires for the control unit to the analogous stereo wires (since I was installing a head unit anyway), and attached the control unit ground and two relay grounds to the metal plate in front of the fusebox. At the other ends of the relay circuits, I tapped the #18 (yellow, for the parking/tail lights) and #20 (red, for the headlights) wires on the headlight switch connector that plugs in below and to the left of the steering wheel, as others have mentioned in earlier threads (you have to remove the lower dash, and then the steering wheel shroud; the latter involves twisting the wheel left then right to remove two hidden screws, then moving the handle that controls the steering wheel angle back to expose a third screw, after which the lower shroud can be removed. I even replaced the DEI switch with a nifty black rocker that fits perfectly in one of the empty switch spots to the left of the steering wheel.

The problem was the windshield wiper control. Neat feature, I thought. After some searching, I found a thread in the Gen III forums that described the left-most (C10) connector in the first picture below--it and the one to the right (the C9, for the rear wiper) plug into sockets below and to the right of the steering wheel. The thread in question said that the white (#1) wire (you can see it's tapped) was the wiper switch control. Hoping the Gen II was the same as the Gen III (the connectors are the same), I tried it.

The result was lights that won't go off as long as the ignition is on. After some searching on the web, I found a repair document that described the wiper circuits, albeit crypically (you can find it by searching for "66 - Wiper & Washer.pdf". The white wire is indeed a control wire, but it's in front of the switch, meaning that, once it's grounded, it's always energized. The obvious solution, then, is to tap the matching wire that's in series after the switch.

Alas, this is where it gets complicated. The #1 wire (white) is the input for the intermittent setting; the output wire (to the wiper motor) for intermittent is is #3 (light green). The low, mist (single pass), and high wiper settings share an input wire, #2, but the output wire for low/mist is the #3 (the same as the intermittent), and the output wire for the high setting is the #4 (dark green--don't ask me why it's a smaller gauge than the other three). In case it's not obvious, all fours of these wires are in front of the motor, and therefore 12+ when energized, which is what you want to attach to the 545t's violet wiper control lead.

Now then, how do we wire this?

1. The first answer that comes to mind is to tap into both the #3 and #4 wires (the outputs), which together encompass all of the speed settings, and attach them to the violet lead. This, however, also wires the high-speed circuit to the low-speed circuit. I'm not sure exactly what this would do, but I'm not about to find out.

2. The high and low circuits share a single control wire, the #2 (red), so why not splice a relay in and use that wire as the relay's control circuit? (This would also entail wiring one pin of the relay to an always-on 12V source.) Well, for one thing, this is a little hard, because the wire bundle is wrapped, and you probably don't have enough play in them to cut, strip, and reattach them, and with the way I solder, I'm not wild about operating a soldering iron that close to all the plastic in my precious car. Probably more importantly, this would only work if the lights were on high or low, and not on intermittent, and the intermittent setting is precisely the one on which you're most likely to forget to turn on the lights manually, since visibility is better in light rain.

3. Tap both the #3 (light green) and #4 (dark green) output wires, and use a 5-pin (SPDT) relay to create the functional equivalent of an OR gate. Specifically, wire the #3 (light green) wire and the violet lead to the normally-closed side of the relay, and an always-on source and the violet lead to the normally-open side, and wire the #4 (dark green) to both of the relay's control-circuit terminals (that is, splice the relay into the existing circuit). I actually was going to try this, but I couldn't find a reasonably priced SPDT rated for 30A (the size of the wiper fuse) at Radio Shack or the two auto parts stores nearest to me.

4. Tap just the #3 (light green) wire, which would cover low and intermittent settings (and also mist, though we might not really want it to), and figure that it'll probably be dark enough for the photovoltaic cell to turn on the headlights if the rain is heavy enough to require the wiper to be on high.

Does anyone have more ideas?