I get regular emails from Parts Express, a great place to get audio/visual stuff, and saw these for outdoor sound systems. I wonder how well these would work on automotive applications?
https://www.parts-express.com/heat-shrink-insulated-waterproof-solder-butt-connectors-(16-14-awg)-blue-25-pack--095-923?utm_source=bm23&utm_medium=email&utm_term=095-923&utm_content=??+Outdoor+Garden+Audio+Must+Haves+??&utm_campaign=200409_Outdoor+Garden+Speakers&_bta_tid=239982896621392152575322640041373706176335660316201822901698285437001196078479099617828515007829434371https://www.parts-express.com/heat-shrink-insulated-waterproof-solder-butt-connectors-(16-14-awg)-blue-25-pack--095-923?utm_source=bm23&utm_medium=email&utm_term=095-923&utm_content=??+Outdoor+Garden+Audio+Must+Haves+??&utm_campaign=200409_Outdoor+Garden+Speakers&_bta_tid=239982896621392152575322640041373706176335660316201822901698285437001196078479099617828515007829434371
They work very well. I bought a pack of them in various sizes around a year ago and they've been awesome. I actually just fixed a wire to the oil pressure gauge in my Monte Carlo with one a few months ago. (Forgot to put it back in the loom and a primary tube on the driver's side header melted the jacket.)
EDIT - Best part is you can melt the solder using just a standard lighter, so it's easy to use them in tight spaces.