wiring and connections are the most overlooked part of installs. very few people ever give them the attention they deserve. i solder and heat shrink connections, which takes much more time and thought. i've finished a connection then realized i didn't like how the wire laid or was routed... and i stop and think "so what"... then i undo and redo. i don't redo my system because it fails or has issues. i redo it to experiment. i improve over time.
while DIY appears to save you money - it doesn't. at the very least it cost you a ton of time. at the worst - your gear fails and you are out more than the cost to pay someone who knows what they are doing.
x2 - systems are not budgeted correctly. you buy subs, buy an amp, and then what? you learn the hard way that it takes another $200 to install it all correctly... assuming you don't have to buy a ton of tools to get the job done.
as an on-call, mobile installer (meaning on-site installs) i have several tool kits and connector kits (large tackle boxes) that are stocked with parts, connectors, terminals, etc. anything i may need to work on cars, boats, homes, for audio, video, data, or general electronics. some installs require i bring 8 different cases for both power tools and connectors, and hand tools. the point is that a DIY install is much more expensive if you don't already have the infrastructure.