1986 Nissan 300ZX Headlamp problems

Electrical problem
1986 Nissan 300ZX 6 cyl Manual

I have a 1986 Z-car & I'm having problems with the left headlamp. Now, I've replaced this headlamp due to the lense being cracked. But the problem is when I turn the car off, my left headlamp will come on automatically. Well, I'll turn the switch on & turn it off. This only helps the problem for a little while. I'll go inside my home & the light will automatically come back on. Well, at first I was thinking that somebody was playing in my car at night time. So I initially started locking the door at night. But last night I just so happened to look out the window & seen my headlamp was on so naturally no one was playing in my car. I'm tired of feeling that I have to babysit my Z-car. I need help. This Z-car is a project that I've been working on for the past eight years & I've got a little investment started with this car. I feel that it is either the dimmer switch or either it is the wires running from the switch to the light?