I have a 1998 e36 convertible and just want to replace the 8" sub behind the back seat with a rockford fosgate 8" sub (4 ohms). The new sub does fit, but wiring to existing sub has two sets of wires. Can I connect positive wire from each set to each other and then to the sub and do the same thing with the negative wire? I don't want to replace the amp or anything else.

That depends entirely on the Ω load of the stock 8in woofer vs. your DVC RF 8in woofer.

Either way the factory amp isn't powerful enough to push something like that worth a damn.