When we bought our Carver last year, the spotlight - also a 291C - would not work. The elevation motor worked OK but not the azimuth motor. The light would come on and off, which confirms at least the switches on the remote were OK.
Unfortunately, when I talked to Marinco this spring, there are not a lot of spare parts available for the 291C any longer, and no circuit boards. As well, the light's base housing was cracked in several locations due to uV damage over the years, so it was time to replace it.
I replaced it with a Guest 500, which is the replacement for the single-wire system, mostly because I didn't want to have to re-run the wiring. Unfortunately, the bolt-pattern of the new Guest lights are larger, so I had to drill 4 new holes in the stainless steel "bridge" on my bowrail.
I can confirm however that the 500 is a wire-for-wire replacement for the 291C. I was able to wire in the 500 without any changes to the existing wiring. That then, was my primary goal.
I must admit though that for $300+, this is a way-overpriced product. In a nutshell, it is cheaply made in my opinion. For instance, one electrical component of the light was simply hot-melt glued to the underside of the housing. Pretty slick engineering for something hanging way out over the bow of the boat that is going to take some punishment.