95 SLS sound system problem

so I had a dvd player installed a month ago at cartoys. When I had the dvd player installed I also had them install the subs and the amplifier. The dvd constantly continues shutting off while I drive or even when the car is idling. The dvd player shuts off and then starts up again by itself. Another thing that I noticed is that constantly now, I keep on getting the "service the vehicle soon" on the display and "anti-theft system problem" as well. I don't know what do to. Cartoys told me that I need an ew alternator but I had it checked and it seems to be working beautifully. The voltage shows that when the car is turned on and the dvd player is working the voltage drops down and then if i turn the the climate on the voltage drops down again to about 12.3 or something. Is this normal? When I play the dvd player and the subs the rmps go up and then down and the lights dim as well everytime the subs hit. I have no idea what to do and it is really frustrating. Any help would be appreciated. thank you in advance.

if your lights are flickering that bad to the bass, it may be that your stock alt is good - just doesn't have enough juice for the car AND the new stereo.

Agreed. Sounds like a high load. Try to run as few accessories as possible until you can upgrade the proper electrical/charging components. Things like defrosters will really drop the volts. I would upgrade the alternator first, then the power wires after, followed by a batter upgrade if its necessary. Most vehicles have an aftermarket higher power alternator option available or worst case have one custom built for your car.