A car electronic shop that shows MECP accreditation can assure you that its technicians are highly qualified. 
If you want to get the best among their staff, ask for a master installer, which is a MECP, certified too. MECP Master Installers are the best men in the field and they were trained to deliver quality performance despite toughest custom works.
If MECP qualification is still not enough to satisfy your picky palate your best bet is to ask around or browse the web for recommendations and comments from other car enthusiasts like you. These people surely have the same experience in upgrading their car electronics and they most likely are willing to share who their master installers are.
Some of the critical car electronics that need to be handled by qualified professionals are:
Back-up Cameras
GPS tracking devices
Auto Finder
Car Alarms
Other car electronics like remote car starter, battery charger, mobile video monitors, and remote entry system are also best installed by a professional car technician.
Your safety and that of your family depends on the quality of your vehicle and its improvements or installed accessories. So despite of your finances you should trust only the skilled professionals to set up aftermarket upgrades to your dearly loved possession. If you hate turning down offers of help from your friends you can ask them for good brands that you should buy or let them suggest which gadget best suit your vehicle and which are far dependable.