On the 201 there are o-rings at the pipes that connect to the heater core. It is possible that you have a leaking o-ring and not a leaking core. The o-rings are accessible without removing the dash. On the driver's side the pipe comes in and elbows into the core up and rearward of the accelerator pedal. Pull the kickpanel out to access it. The passenger side pipe can be accessed after removing the glovebox.

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff