Cleanest way to splice automotive wires in car?
The "cleanest" way is a tin soldered heat-shrink sleeved connection.

Other ways must guarantee a gas-tight connection between the wires contact surfaces, which can corrode over time.



I have not tried these IDC crimps, but with the plastic threaded lock nut to seal the IDC crimp under pressure, I would support use of these but never any other crimp connectors not rated for automotive use with reliable crimp force and moisture block.



https://www.posi-products.com/instructions1.html