I was afraid you would ask. It will be a bit long but if you do as I suggest, you can pretty much be sure to find the problem.
Lights are dimming? This indicates voltage is low and this is to be expected to some degree on an older bike. Can be a combo of any of the things I note. The thing we will do here is to verify the charging system is doing what it is supposed to do.
Step 1. Check the battery. They are made cheap and sometimes sulfate quickly. This means the plates attract goo and it chemically bonds with the plate and doesn't allow a good charge. You always verify the battery FIRST. Best way to do it is to use a load tester which is a fairly expensive gizmo OR use a hydrometer which is a cheap gizmo. The hydrometer is basically an eye dropper with colored balls. You open up the cells on the battery, put a small hose on the hydrometer and suck fluid in. The balls will either ALL float which means the battery's cell is OK or some of the balls will not float which means the cell is dying or dead. If any cell is questionable, replace the battery. When you charge a battery use a 1A trickle charger and charge for at least 12 hours. Don't use a car type charger as the amperage is too high and you can ruin the battery.
Step 2. Battery is now OK! Check your stator. Don't know what kind of bike you have but MOST KZs have a 3 wire stator. The wires will be three yellow wires or 1 blue, 1 pink and 1 yellow. These are three legs of AC coming off the stator and plug into the regulator/rectifier. Unplug the connector that connects the stator wires to the reg/rec and start the bike. Use a multimeter set on VAC in a scale that can measure 10-100VAC. If you put your probes on the A and B leg, you should see about 40VAC at 4K rpm. Same with the B and C legs and the A and C legs. Test all different ways and you should get consistent AC voltage on all three tests. If you get a little more or a little less than the value I came up with (40VAC) that is OK because I can't remember the actual number and that number varies by bike but if you only get 15VAC per leg, the stator is weak. If you don't get anything when testing on one pair of wires, the stator is bad. If you get the 40VAC on all three tests, you are OK. If not, replace the stator.
Step 3 - OK you have good AC voltage on each test so check your reg/rec. Whether you have separate or a combined reg/rec doesn't matter too much at this point. The best way to tell if the reg/rec is working is to reconnect the stator and check voltage at idle at the battery terminals. You should see about 12.5VDC so put your meter into VDC scale and try. Then rev the engine to 4K rpm and check. you should see about 14.5 VDC. If you do, move on and if not, we will need to do some testing on the regulator or reg/rec. You need to have these voltages at the level I noted or the battery won't charge good. If you have these voltages, move on...
Step 4 - All the components are working OK... now get into your wiring harness and take apart EACH connector, clean with spray television tuner cleaner and dab dielectic grease on the connector pins. Open up the left switchgear and clean the hi/lo switch. Note there are red/yellow and red/black and a black/yellow wire harness that connects to the main headlighht harness. These wires connect to the headlights. There will usually be a fried connector in this area. Damaged wires cause higher resistance and will cause voltage to drop.
