Since the cassette deck died on me long ago, I thought it would be an ideal space for my dock. If your cassette deck still works and you still use it, don't worry, this mod will not affect the operation of your cassette deck.
Stuff you'll need:
-3.5mm aux cable
-Cassette tape that you don't need anymore (the type that are screwed together are much easier to work with)
-Some adhesive Velcro
-Various screwdrivers depending on which ones your boombox needs for disassembly
-Soldering iron and solder
-heat shrink tubing
Step 1: Take apart your cassette tape and get it ready for your iPod
Most cassette tapes will have 4 or 5 little Phillips screws on the back side. Take these screws out and separate the two halves of your tape. Remove all of the magnetic tape itself (you can just take the two reels out and all of the tape will come with it)
Then just trace an outline of the iPod you are going to use onto one half of the cassette and mark a spot to put a hole on the other half for your aux cable to snugly fit into.
Use a hobby knife or exacto knife to cut the plastic out where you outlined your iPod. This may take a little bit of work. Then just get a drill bit that is slightly smaller than the diameter of your aux cable's connector and drill out the hole you marked.