I havent personally installed a Tsunami into the Athearn Challenger, but I have seen the innards of the tender and the change wouldnt be that difficult (I have installed many Tsunamis in various locos). When you remove the tender shell, you will see 2 PC boards and the speaker assembly on the roof of the tender. Clip off all the wires that run to the decoder. Remove the large PC board (Leave the small one on the front of the tender) by removing the little screws that are holding it on. Clip off the wires going to the speaker and throw the decoder across the room (to make sure you wont be tempted to use it again).
On the front PC board you will see some soldering pads with wires comming off of them. You need to find out which one is for the motor, pickups and light. once you are sure you know which pad is for what function, then you can feel free to solder the wire leads directly to the solder pads.....orange/grey for the motor, red/black for the pickups and white/blue for the forward light.
The last step is to solder the wires to the speaker assembly.
Test the locomotive on the test track, if you hear sounds, have light, and have motor control, then you are golden. Reassemble the tender.
Time to program CVs.
Go to the Soundtraxx web page and download the manual for ALL the cvs in the Tsunami. The first CV you need to set is the speed table. Get the loco moving like you want. Then set the CV for 'articulated mode'. Then set the CV for CHUFF rate to match 4 chuffs for revolution. Then set the CV for slip rate (to make your drivers go in and out of sinc).
Beyond that set the loco to your liking. Choose your Whistle, Bell rate and various other Tsunami goodies.
Hope this helps.
David