Reddit mentions of Genuine Fender Replacement 1/4" Internal Speaker Cable, 003-8566-049

It looks like parallel wiring so...maybe not cheapest way (if you have a soldering iron then definitely not cheapest) is to hook this up (I bought this for a 2x10 cab I built after I found the speakers had only 2 tabs not 4):

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https://reverb.com/item/5704954-earcandy-2x12-guitar-amp-speaker-cab-parallel-wiring-harness-w-jack-cup-hardware-already-soldered

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If you don't mind it (the connection) being outside you could use a spade bit to make the hole in the lower panel and put this in...you still need to make the internal speaker plug so I guess soldering iron is required.

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So you are connecting this to the white and black wires before it goes to the speakers by cutting that wire coming out of your amp and solder the spliced end of something like this...

https://www.amazon.com/Genuine-Fender-Replacement-Internal-003-8566-049/dp/B07HY7562T/ref=pd_cp_267_2/135-2771638-8671050?_encoding=UTF8&pd_rd_i=B07HY7562T&pd_rd_r=964ee13b-9535-46d0-9d04-770e20160a5c&pd_rd_w=B1hC6&pd_rd_wg=8kObF&pf_rd_p=0e5324e1-c848-4872-bbd5-5be6baedf80e&pf_rd_r=9NQK3BR8T94XD5HYGYZ8&psc=1&refRID=9NQK3BR8T94XD5HYGYZ8

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I can double check this tomorrow and try to think it through so don't order anything yet.

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***You could also just buy something like this and then solder the wire parts that you leave on the speaker clips to this (about 14 dollars cheaper than the ear candy thing on reverb) :

https://www.amazon.com/Black-Speaker-Plate-Round-Switchcraft/dp/B0716NH7XZ/ref=sr_1_2?keywords=input+speaker+jack+guitar+plate&qid=1573768232&sr=8-2