The only guarantee is if you use a hermetically sealed butt splice. Otherwise the wire is exposed and will oxidize over time. No, heat shrink alone is not hermetic but they do make heat shrink and butt splices that do result in a gas tight seal after heating to melt the sealer. That being said, many people get by with simple butt splices every day and they can last the life of the vehicle. I’ve had some underhood splices fail after many years but my environment has a lot of road salt and long winters…