Sounds like a freeze plug, or soft plug, or frost plug, etc.... everybody has a different name for them. Should be able to get them at just about any parts store, usually quite cheap.
I've never changed one, but it seems to me that they just tap in
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1989 7MGTE R154 Targa Top White Package w/Blue Interior
Driftmotion 57trim CT26, Upgraded Intercooler, 2.25" Hard Pipes, 3" Turbo Back Exhaust with Test Pipe, HKS VPC w/GM 3 bar MAP sensor, APEXi S-AFC, Walbro 255lph fuel pump, ProSport EVO Series Boost / Fuel Pressure / AFR gauges, Lotek a Pillar Gauge Pod, HKS Type 0 Turbo Timer, Eibach Lowering Springs, Tokico Illumina II TEMS struts, custom powder coated Motegi Racing TrakLite wheels
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