Stereo system
 

Looking to install a modern stereo system in my 1990 Mirage 257. Anyone have any tips, pointers, warnings? It currently has the original kenwood deck and speakers which aren’t working at all. Thanks for any insight.

Without some parameters your asking a wide open question. The biggest change in your system is going to be that everything has Bluetooth , amps have become more efficient ( class D)and led lighting has become a thing . Are you just replacing like for like or going crazy?

Well I bought a Rockford Fosgate setup(2 10s, 6 6.5s, 5 channel amp, receiver). Just don’t want to hack up my boat. It’s old but in good shape with all original upholstery. Not very savvy in the electrical department and just wondering if I’m going to hit any road blocks there. Also was wondering if and how to possibly add a 3rd and or 4th battery(if that’s even possible?)

I would start by swapping the speakers out , mount the amp and build a box for the sub. If your hesitant about cutting the boat up you can listen to the system for a while then decide if the box works or you would like to permanently mount the woofer somewhere. Adding a dedicated house/ stereo battery is rather simple using a constant duty relay or a battery isolator that would allow you to run the stereo battery down without worrying about killing the starting battery.

Do you currently just have (4) speakers...or maybe only (2)? Installs like this can be fairly straightforward...or end up requiring custom fabrication to make it look good. As carnutsx2 has said, going "like for like" as much as possible is the easiest route, but if you are putting bigger speakers in place of smaller, it can be pretty daunting to open up the hole (or add a hole) in fiberglass. I highly recommend a tool like a ROTOZIP for this, but regardless of what you use to do it, there are good ways and...bad...ways to do it. I've seen total hack-jobs that just made you roll your eyes, sadly. Cutting fiberglass has its challenges but isn't that bad if you know some basics.
One thing you will need to do is to run high amperage wires if you are pushing very many watts at all. You can't just use what was there if it wasn't a robust system to start with...typically you'll want, at a minimum, 8 gauge oxygen free copper wires...larger is better. I personally wouldn't go less than 6 gauge, the last install on a boat I did used 4/0 cables that went into a distribution block, that had various 'tap' sizes coming from it.
You must maintain the proper ohm load...matching your speakers to the amp. You mention you have a 5 channel amp but you also reference (6) 6.5" speakers plus the subs; one of the 5 channels is for the subs and then you'll have front and rear (left/right) for the other (4) channels...if you drive (4) of the (6) speakers off one pair of these four channels...for example you have dedicated front speakers coming off the front output of the amp, and then you have to run the (4) mid-cabin/rear speakers off the rear channel of the amp. You can't drive (4) four ohm speakers that way. If you bought this Rockford 'system' it may be set up to do this with special ohm loads on the speakers...anyway, something you should be aware of.
You'll have (4) wires going to the head unit from the boat's main harness typically: red for power, black for ground, blue for amp turn on, and orange or yellow for constant power. Those will likely be small wires in the existing harness...no more than 14 gauge wires. It is the amplifier(s) that you'll want to run dedicated power with big cables off a battery, preferably a 'house' battery like carnuts references. If you don't have dual batteries already, it would be a good idea to do this and have a battery switch.

Thanks for the good info, I currently have 6 speakers. But only upsizing 4, going to replace the two in the cuddly with new 4” speakers being I do not have much room to cut the holes bigger.

Most ( I won't say all ) Rockford amps will easily run 2 speakers on a channel at 2 ohms. I assume that you will run the cabin speakers on 1 and 2 channels with the amp switched to high pass around 120hz , the 4 others on 3 and 4 wired in parallel so the amp sees a 2ohm load ( more power ) switched to high pass around 90hz . Depending on what subs you have you can wire them in the 5th ( sub) channel so it sees a 2ohm load . To cut bugger holes if you can I would use a hole saw . Screw a piece of plywood behind the existing speaker hole . This gives you something for the arbor of the hole saw to locate the new hole. As far as power wire goes, look at the current draw of the amp at max power then work backwards by measuring the distance to the amp . You can find the chart on line. Don't forget that DC voltage is a 2 way street, you need a good ground and may need to run equally sized ground wire to the amp. Sub location and enclosure design is another issue .
don't forget a circuit breaker or fuse
Good luck

I'd run the cabin speakers off the head unit and reserve the amp for everything in the cockpit. That's where you'll spend the majority of your time anyway especially on a 26 foot boat. My 311 is set-up the same - the headunit does perfect in the cabin. Just my thoughts.