Reddit mentions of Infinity REF6520CX 2-Way Component System ( With Edge-Driven Textile Tweeters - Pair,6.5" 540W Reference Series ), 1 Pack

You're not going to be able to keep the cassette player. I'm sorry to break it to you, but for the same reasons you can't buy an 8-track player, you can no longer buy a cassette player. As pdb1795 said, you can always get a new stereo with CD/AUX, then use a walkman plugged into the AUX to play your tapes. But I suggest converting all your tapes to MP3 ASAP so they'll stop losing sound quality everytime you play them. Plus you'll be able to play those MP3s without a walkman on your new stereo.

​New stereo (all harnesses and brackets included for free)

Amplifier

Wiring

Front Speakers

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Subwoofer depends on how much bass you want and how much trunk space you're willing to sacrifice. It also sounds like you're getting it all installed professionally, which is a smart idea. So figure at least $250 for labor, likely more like $300.

Thanks for the tips! I found an adapter that Crutchfield thinks will work and found these for a decent price on Amazon but I still worry about getting them to fit since I won't be doing the install... I guess I shouldn't worry since Crutchfield is pretty good at what they do but I don't want to have whoever does the install to not be able to complete it because they don't fit and then still charge me a labor fee.

Thanks for your help, I have definitely learned a lot. I am slowly coming out of the rabbit hole of audio setups and facing reality. I went all crazy looking at DPSs and LOC and powerful amplifiers and it was fun but it was also overwhelming. In reality I don't need "superb" sound, I would be satisfied with "fairLy good".So I have come down to reality and now I am thinking about the below setup based around the Kicker Key amp. Im thinking 45W should be plenty of power compared to stock and also the hands free DSP setup is gonna be a godsend. I think I found a decent deal on older model Infinity speakers below.

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{FRONT & TWEETER} (planning to use the passive crossover for the tweeters)Infinity REF-6522EX Shallow-Mount 6-1/2 Inch Coaxial Car Speakers ($43.99)https://www.amazon.com/gp/product/B01FUZV7ZG/ref=ox_sc_act_title_1?smid=A1DXN92KCKEQV4&psc=1

{BACK}Infinity REF6520CX 2-Way Component System ( With Edge-Driven Textile Tweeters) ($83.95 )https://www.amazon.com/gp/product/B01E62D7BQ/ref=ox_sc_act_title_2?smid=A1M2AR6046TFJG&psc=1

{AMP} (The speakers are 3 ohm, though the Kicker Amp is 4 ohms, but based on other threads it should be all right)Kicker KEY180.4 ($199.99)https://www.crutchfield.com/p_206KEY1804/Kicker-KEY180-4.html

{AMP WIRING KIT} ($14.99)Crutchfield CK12https://www.crutchfield.com/p_007CK12/Crutchfield-CK12.html

{SPEAKER BRACKETS} (not sure if I really need these as it says that I will need to drill new holes for the screws anyway)Scosche SAC-656 Multipurpose Speaker Mounting Brackets (2 x 12.99)https://www.crutchfield.com/p_142SAC656/Scosche-SAC-656-Multipurpose-Speaker-Mounting-Brackets.html

{SPEAKER HARNESS} (2 x 9.99)Metra 72-7300 Speaker Wiring Harnesseshttps://www.crutchfield.com/p_120727300/Metra-72-7300-Speaker-Wiring-Harnesses.html

{SUBWOOFER} ??(I am yet to look - though I figured I can do this in two steps)

Total $388.88 + tax

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Questions:- What do you think?- I should still be able to control the volume using the stock HU volume knob correct?
-What dampening do you recommend as a basic thing while I have the car open?

As far as I can tell, you can fit 6 1/2" speakers in the front doors with a bracket. That ' 3 1/2" ' speaker you're looking at in the door might actually be something odd-ball like 2 1/2". The rear deck is indeed 6 1/2", not sure where you got 4" from.

If you have the Harman Kardon audio system you might have a couple shallow-depth 6x9s in the trunk under the rear deck, but even so I'd leave those alone.

Start with the front. I'd go with this. The woofers are 3 ohm impedance, and your OEM woofers are either 2 or 3 ohms. If you get something that's 4 ohms you're going to lose some power. (This is irrelevant if you're doing something with the head unit / amp though)

Edit: You can snag that Infinity component set on Amazon for $70 cheaper, but I think you'll need to buy the mounting brackets separately