Even so the speaker jacks are for output so that's a no go, And the mike jack I would think is not stereo or the correct impedance so that's highly likely a no go as well. I'm afraid those DIN jacks where the only proper input and they are going to be the wrong impedance too. 
 
I see old gettos in the Goodwill all the time for $12-$24. Just find one with RCA input jacks. 
I have a set of small computer type speakers on my "B" channel of the Technics amp I use for driving my monitors. It really doesn't tell me much, the sound just goes from great to bad...  so I rarely switch to them when mixing. I still find it better to put the songs on a USB stick and go play them in the car or the living room. 

Cactus Music
Even so the speaker jacks are for output so that's a no go, And the mike jack I would think is not stereo or the correct impedance so that's highly likely a no go as well. I'm afraid those DIN jacks where the only proper input and they are going to be the wrong impedance too. 
 
I see old gettos in the Goodwill all the time for $12-$24. Just find one with RCA input jacks. 
I have a set of small computer type speakers on my "B" channel of the Technics amp I use for driving my monitors. It really doesn't tell me much, the sound just goes from great to bad...  so I rarely switch to them when mixing. I still find it better to put the songs on a USB stick and go play them in the car or the living room. 

I would seem that's the case. The best I could do was record to a tape (I found one), and after that rewind the tape and listen to the song (which btw sounds very bad). Must be quite old boombox'es I found. As you say, I need to find a one with RCA inputs, or hack these old ones so they have aux input.
Thanks for all the tips though!

pilutifulThe best I could do was record to a tape (I found one), and after that rewind the tape and listen to the song (which btw sounds very bad). Must be quite old boombox'es I found.

did you demagnetise the heads and clean it up before use?

Better yet, Buy a new getto with a USB input. Most new gettos have this feature so you can play MP3 and every getto I have around has a mini AUX input too to accommodate iPads and cell phone MP3 playback. 
 
If your goal was to proof mixes on other systems converting to MP3 and using modern consumer playback gear is a good idea. Burning a CD an playing in the car is a good idea. Using a tape is a bad idea as that won't be happening anywhere in the real world. The sound will be different because of the tape media, so that tells you nothing other than re mixing for a tape ;?