Poor radio reception has a few typical causes in the MkIV (and a search will reveal lots of good info), but corrosion within the antenna base is the most common. The antenna itself generally outlasts the base, but once the outer rubber coating on the ant starts to wear, it's a cheap and easy replacement.
Use only an OEM base and antenna (unless also modifying the system), as the aftermarket ones will have you back in there doing the job over again sooner rather than later.
There's no harm in replacing both the base and ant as wear items, but I'd suggest troubleshooting the system first, so that you don't miss a cause and have to go through the process again (trunk items out, C pillars and trim off, headliner lowered, wire released, bolt out, base off, etc.). A few radio reception issues include the wiring from HU to ant base, and some of this is accessible with the headliner lowered, other times water intrusion is an issue, also good to know if the headliner is lowered. Lastly, there are a few other maint opportunities with the headliner lowered or removed (especially if you have a sunroof), so my primary rec is a thorough search of the threads here. HTH.
Although the differences on the outside are notable, the quality of the little circuit board inside is very different- as is the "weatherproof" sealing. Clicking on the pic may lead to a few more pics from the swap. No pics of them disassembled, as other threads can show you the difference in quality of the OE base vs aftermarket.
