If you think the new Mexico is somewhat overpriced, why not consider one of those refurbished stereo Beckers, such as the Europa (I or II) stereo, or even the Grand prix stereo with optional aux-in jack (Königs)?
The external jack is connected to the back plate of the unit (DIN-jack adapter) and needs no drilling.
Of course, these oldies shouldn't match the sound quality of the modern Mexico, but the limits of the whole system will still remain the speakers , specially their questionable location in the kick panels, wich is-in my opinion- anything but ideal for high frequency response.
I have a Grand Prix stereo (with stock external amp) and a pair of footwell speakers (2 way all-in-one Pioneers) .Sound is boomy, trebles missing.
One solution might be installing tweeters in the center dash well, but this would require a discrete two-way system .
Remember anyway that this is only for low-speed running, as the exhaust, wind, and soft-top noise will quickly overcome the radio at usual speeds...