The stock CR-718 cassette stereos used in the later 940s are not known as reliable or repairable. I've gone through two in each 940 before giving up. Generally it's the panel and/or tuner that locks up. Crackle can be the speakers failing unless associated with turning the volume knob, then it would be a worn volume rheostat. Use the fader and balance control to see if you can isolate the crackle to one speaker.

I've put mid-priced Sony MEX stereos in both cars and am very happy with them, both for sound and looks in the dash. I especially like the improved panel layout and bass boost with the newer MEX-N5300BT model. You can change both the the LED colors and brightness to suit your tastes and even match the rest of the dash lighting. They have a CD player with a nice little remote control. Anything much higher end than that would deserve a speaker upgrade as well. You tie the 940 dash tweeters in parallel with the front door speakers. Lots of AM and FM presets. These are bluetooth handsfree phone capable (suopports two phones) with a separate microphone (that I attach to the blank switch plate next to the ignition switch). They also support bluetooth audio devices and have both USB memory stick (my player of choice) and audio player device (iPod) ports as well as Pandora and Sirius satellite receiver capability. The faceplate pops off for safe storage when not in use if you wish. Crutchfield is where to get them as well as the needed 700/900 dash DIN slot adapter and radio harness adapter. Installation is fairly straightforward. It will take time to get used to the dial menus used in all newer radios as there are numerous settings and options.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now