Speaker ground?

I think your issue is actually something different. I believe you may be having an impedance issue. I do not know what the impedance is of the original radio but standard is 8 ohms. The speaker is my 65 Barracuda ran a single wire to the radio and one to ground. If you wired both speakers by splicing them as you say then you are actually doubling the impedance. This does 2 things it causes the amplifier in the radio to create more heat and second less sound to each speaker.

I could be wrong and my car audio knowledge is getting aged but try hooking 1 speaker up and see if the volume is louder.