How can aluminium cables be lengthened for automotive environment?
You can solder aluminium, or at least certain grades. I can solder to cooking grade aluminium foil for instance.

The trick is to buy a specialist multicore solder, with a particularly nasty aggressive flux in it. I find it easiest to 'tin' all the surfaces, then use conventional solder to complete the joint. It needs some practice to get the solder to take, especially getting the temperature right. I don't know whether it is still available, I bought mine 3 decades ago, and still have some left!

One alternative is screw clamps, another is crimp. I'm not sure either would be too robust against possible corrosion of the mating surfaces, unless scrupulously encapsulated after joining. Both would be larger profile than soldering.