He's talking about the antenna base, not the mast. Well - both actually - but masts that fit the Isuzu base are a dime a dozen, while a reasonably priced Isuzu base is near impossible to find. Go ahead and call Merlin about the price. I bet you won't be buying OEM!
David - try doing an eBay search for "Ford Focus antenna base" - or VW Jetta, or Toyota Yaris... maybe one of those will work and they're 1/5 the cost of OEM. Hard to tell just by looking at pics though - need dimensions to know if it will fit properly. My antenna is in the same condition as yours. The plinth is pretty much falling apart. I dread replacing it because the headliner will have to be removed. What a PITA. So to stave that project off as long as possible I used black RTV to seal up the cracks and keep water from getting to the headliner. Needs to be replaced though....
....So if anybody out there has already found a good substitute, chime in and let us know!
Thanks!