Not knowing your budget, what you want your music source to be, or the sound quality you want, it's a guessing game.
An option is to mount a slim EQ like this under the dash. Leave your orig radio in place for appearance sake?
Find an EQ that has a volume knob, like this one. (most have it, but not all)
Some also have a 3.5mm input jack on the front panel.
Connect your MP3 player to it with a 3.5mm cord, or a 3.5mm-adapter into the RCA jacks in the rear.
You'll have volume control from both your MP3 or the EQ (although that one will be a long reach).
If you are playing via Bluetooth from your phone, you could find some type of BT adapter to mix into this.
I have this little EQ and it seems OK, and CT Sounds has a decent rep on amazon.
The 6x9s have a silk-dome tweeter and should have non-harsh highs.
The amp is 4-channels, so if you ever wanted to add front speakers or a wooferbox. (a single 8" is sufficient for most of us old guys)
If you want to cut cost, a Boss 2-channel amp should suit well.
If this route interests you, I could make more suggestions.
If you are wanting to listen to AM or FM - I have no more suggestions.
https://www.amazon.com/dp/B01N9UK4Z...SDVCAE7S&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it
https://www.amazon.com/dp/B01LZ6D96...SDVCAE7S&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it
Oh, and a wiring kit.
https://www.amazon.com/BOSS-Audio-A...-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1