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.
Step 2: Wire Your Aux Cable
Then unscrew the case of your boombox and separate the two halves. Most of these old boomboxes have an aux/line in somewhere. Once you have the case separated, find the backside of the aux L and R channels and solder your red and white wires to them and also solder the grounds that you twisted together to the ground on the backside also. Remember to use heat shrink also.
Step 3: Route Your Aux Cable
Step 4: Cut Some Velcro for Your Cassette Tape and Deck
Step 5: Put Your Tape in and Mount Your IPod
I've entered this in the 'Vintage' contest, so if you like it, please vote!