While the AT-300 has long been discontinued, it is still the "standard" 
that Kenwood radios  interface with.  Other external auto tuners will 
emulate the AT-300 when interfacing with Kenwood radios.   I'm fairly 
sure that if you can figure out the pin outs from the tuner cable, you 
can connect it to the molex jack on the radio  and then just use the 
radio's tune button to auto tune.    There may well be some pre made 
cables out there if you dig around.   If you check the e-ham  product 
reviews, there are guys who will do custom interface cables at 
reasonable cost.

There is also a trick of putting  a 10k ohm resistor between the center 
2 holes of the molex jack on the radio. While this doesn't control the 
auto tuner  automaticly, when you hit the tune button on the radio, it 
bypasses the internal tuner,  switches the radio to CW and lowers power 
to 10 watts.  Then you tune the tuner, and when it is matched, hit the 
radio's tune button again to return to original mode and power setting. 
   I've never done this and am going from memory, so confirm from 
another source before trying this.

73, Steve  KW3A