First, a word of warning. A car battery stores a huge amount of energy, and if that is allowed to escape in an uncontrolled manner the consequences can be dire. Don't meddle with the electrics unless you know what you're doing. My own car bears the scars of such carelessness. When I bought it, there were a couple of pointless paintball stickers on the bonnet. I took them off, to discover that one of them was concealing a round black mark, about an inch in diameter. When I found a similar mark on the other side of the bonnet I realised what had happened. Someone had fitted a battery that was too tall for the car and shut the bonnet....
Having said that, SJ electrics are not particularly complicated and fairly easy to modify if you follow a few basic rules. Keep it neat. Don't rely on a wire's insulation to protect it from damage - always protect it with tape or sleeving. I use either a springy plastic spiral sleeve or self-amalgamating tape. Always fit a fuse to each new circuit that you add. Never use those hideous Snap-Lock "connectors" to tap in to existing wiring - they are nothing but a bodge for people in a hurry. Buy yourself a crimp tool, or learn to solder. This is what the wiring looked like on my car when I bought it - horrible, isn't it?
The first thing you need is a wiring diagram. Don't bother looking in the Haynes manual, you won't find one for an SJ in there. Click here for the correct diagram for a European specification SJ410 in PDF format.
Before adding any extra gadgets you need to think about how to connect them up to the battery. You could get a fuse box from a scrapyard car. I chose to make a junction box and use inline fuses for each new circuit. This is what it looks like with the cover off. Remember that the wire supplying the junction box must be rated to carry the total current of all the new circuits. I used two pieces of 17 amp cable in parallel.
This is how to beef up the horn with pair of twin tone units or air horns. Here is the original horn circuit and the modified one with the new wiring shown in red. The switch allows you to change over from the original horn (for road use) to musical air horns.
Driving lights are easy to fit. To stay legal, these must be wired to come on only with the main beam. Here is the circuit, again with the modified wiring in red. I've left off the side lights and dim-dip wiring because they aren't relevant here. The rating of the inline fuse will depend on the power of your bulbs, but should be at least 10A.
