There is with and without Infinity sound system,,not sure which you have.
From my experience, factory amps failures are not uncommon on Mitsubishi and Bose. Often, the systems are integrated into other systems such as steering wheel controls so owners are understandably reluctant to go to an after market system.
This aside, the after market systems (even budget models) are more reliable and sound much better than your factory system. Now, they also have wireless remote on most which makes one miss the steering wheel controls much less.
The most common cause of amp failure is continually operating at a very high volume or even worse, with a blown speaker somewhere.
As the amp is likely located under the RF seat, you may want to make sure that it is not getting wet or otherwise contaminated.
The solution? Factory amps are way overpriced and with only a 12 month warranty, just not worth the expense.
Here are a couple of links for used amps. Several around $70.00.
Car-part.com lists all radio parts in the radio section.
Also, here is one on Ebay for a 98. I'm sure the seller can take pics of the connections and your Mitsu dealer can advise if compatible.
If it were me, I'd spend about $300 and get a new, great sounding after market system.
Good luck and let me know if you have any other questions or concerns.