Its not the signal to noise ratio that is affected by a splice, its the standing wave ratio. The quality of the splice can reduce the impact to almost zero if the right spice is used correctly. The issue is trying to keep the Characteristic Impedance (Zo) the same as the cable. I.e., use a 75Ω splice on a 75Ω cable. I good splice will usually require a heat gun.
I agree the best solution would be to start over with a new antenna. I’m guessing that the antenna is on the rear fender and the so called tech didn’t want to or didn’t know how to snake the cable from the back of the car, so he just cut and spliced, very poorly at that.