installing a line-in in my new 240!

[s1kboy]

Hello all! I just got a 1984 Vovlo 244 GL and I'm very happy with it! I want to hear some music! I don't want to replace the head unit so I have been told that I may be able to wire in a line-in such as this one or this. Does anybody know about these? Anybody have any experience with them? Installation?

I want good quality and I am willing to do the work myself. Not trying to spend a lot of money. Now I am looking to play music from my phone by plugging in an auxiliary cord so this is what I have found. Any advice,tips, tricks are appreciated! Thanks