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A map light was not available in 69. The inside rearview mirror mounted map light was first offered in 1973, and continued to be available through 1976.
73-76s with a map light, used an auxiliary harness, to connect the map light to the dash harness. The auxiliary harness is available for less than $20, but unlike a 73-76, which had provision for the auxiliary harness, you'd have to modify your 69 harness to connect a map light harness to it.
I just took a look at a 74 AIM I have. I assumed that the map light harness plugged into a connector in the main harness, but Easy Mike's right. According to the 74 AIM, the harness plugs directly into the fuse box, in the receptacle marked "ACC" (accessory). By the way, all 68-76 inside rear view mirrors use the same mounting bracket. Knowing all that, I would think that it would be a pretty easy swap.
I installed the C3 map light in my 1970 back in 1973 because I liked the wider mirror in addition to having a new Corvette option. I connected the power wire to the IGN terminal of my fuse block so the mirror light would only work when the engine was running.