I just replaced mine last week with the replacement recommended in previous threads here - the JL Audio 6W0. I found mine via a Google search, and the first place I tried to order it from ("techronics.com") was evidently pulling a bait and switch, as they didn't really have any in stock, and tried to sell me a mid-bass speaker. I found the sub in stock at "woofersetc.com" for $94, and CA tax and shipping totaled to $112. Only issue I had with them is that I didn't know whether I was getting the 4 ohm or 8 ohm version (the 6W0 comes in both impedances) - they don't offer the option to specify which one, so I took a chance. The stock speaker is a 6 ohm, so probably the 8 ohm is a better choice. However, I ended up with the 4 ohm version, and it is so far fine. As it is 33% less impedance than the stock speaker, it might over-tax the amp, is my only worry.
If you get this replacement, it comes with two nice rubber pieces that you have to remove for it to fit in the Viper mounting location - one around the magnet and one on the face - takes 5 seconds as they just snap on/off. Then pop the grill off of the car, remove the existing speaker, remove the wiring harness from the stock speaker and attach it to the new speaker observing the polarities. As I wanted the "JL" logo to be correct through the grill, I drilled new holes for the mounting screws (at 45 degree angles to the original mounting holes).
All in all, about a 15 minute replacement/upgrade.
